I am a Catholic interested in converting to Judaism. I was told online by a Rabbi that Orthodox is the only true way to convert. Is that true? I am looking for advice and guidance. Thank you
Depends what you mean by "true".
For millennia, Judaism believed roughly in the same things as what we now call Orthodox Judaism, and abided by roughly the same rules. So in that sense, yes, Orthodox Judaism is the only true way to convert: other conversions are not to Judaism.
However, if you want to convert to them, then by all means do so. There's nothing wrong with doing so that I know of, and you will be joining a large community of Whatever-Movement-You-Choose "Jews" who will, I trust, make you feel welcome and teach you about their version of Judaism.
Be aware, though, that you will not be considered Jewish by Orthodox Jews, with all the privileges and responsibilities thereunto appertaining. (Which is why I put quotation marks in the preceding paragraph.) Not that there's anything wrong with that, but, for example, no Orthodox Jew will marry you.
 But I'm no rabbi or expert in Jewish law.
 Being Jewish, as Orthodox Jews define it and certainly as I'm using it in the phrase that led to this footnote, means having a certain technical legal status. This is not an ethnic or self-identification status.
The definition of a Jew according to halacha (Jewish law) is
- Someone whose mother is Jewish (according to this definition)
- Someone who has converted according to halacha.
This is the first recursive algorithm known. The start point was the revelation at Sinai when all those who received the Torah converted at once.
Only the Orthodox conversion will make sure that you learn all that you need to do and will be accepted by everyone. If you convert in some heterodox movement (such as reform) you will be told that you do not actually need to follow the Torah (such as keeping the kosher food laws or not violating the Shabbat).
You should make sure that the bait din you use is accepted by the Israeli Rabbinate in order to avoid problems that could arise once you have converted. The Rabbinical Council of America could probably point you to someone in your local area who can help you as we see in this article
You should find a rabbi who is an expert in guiding and working with converts in order to make sure that you are able to be taught properly and thoroughly. I do not know where you live, but you should try to find a local Orthodox rabbi who can speak with you and help with recommendations. It is always better to have someone who knows you personally that you can speak with, even if he is not the one teaching you.
May you have success on the path you are considering. While you may find difficulties as you go along, the results will be worth the trials.
What he likely meant by true was as defined by the Torah. There are other movements in Judaism that use their own ideas or outside ideas to be interjected into Judaism but orthodoxy as it defines itself is the one that uses Torah sources to define itself. This can be confusing for someone not currently an orthodox Jew as within Orthodoxy there are many different paths to follow and within those paths not everyone is 100% consistent as the human condition makes people (whether they realize or not). Something to consider is that Judaism isn't a religion. It's not an all in or nothing type of a deal when you are born Jewish. For such a person it is within the fabric of their being to be Jewish. They then can chose how to be involved in their Jewishness (hopefully by keeping the Torah as G-d wants us to). As for someone who converts. There is an idea that this person had within them a spark of a Jewish soul which only needed to be nurtured in order to be fully revealed which is done through the conversion process and by living a fully Jewish life according to the Torah. Like the Talmud says... a convert who converts. That is what was meant by true way to convert. Now as for the practical process of how that happens...
As for how to convert there are a few steps... 1) Acceptance of the whole Torah in belief and in practice. Before one finishes there are some comandments one will not fully observe such as sabbath and tefillin. (A rabbi can explain to you the details when you talk to him). Most conversion Rabbis require a period of learning as well. Strictly speaking that isn't part of the conversion process but the Rabbis want to set you up for success so that way when someone does convert they know how to live as a Jewish person according to the Torah. 2) Bris Milah/circumcision. (Males only) If you haven't had one you will need to get one. If you have someone will need to inspect to find out if complete and no foreskin remains.If complete then only a small drop of blood is required. 3) Mikvah. After the circumcision one showers, cuts nails, removes all dirt and anything that would come between the person and the mikvah water. Before going in the Rabbis will usually ask once more if you are sure if you want to go through with this as the Jewish people are persecuted and it will not be easy to keep all of the mitzvas. If yes you go under once then you say a blessing and then go under once more 4) Sacrifice. Since the Beis HaMikdash currently does not stands were are not permitted to bring sacrifices but in the time of Moshiach we will once more and all converts will bring their sacrifices then
If I asked a Catholic priest about converting to Christianity, he would also say Catholicism, as opposed to Protestantism or Eastern or Coptic is the way to go. This website is (Jewish) orthodox and naturally we would agree that Orthodox conversion is the only type we recognize but if you go to other movements, they will say their way is true also. In my humble opinion, reform and conservative put huge emphasis on "tikkun olam" (fixing the world, acts of kindness etc) and if that's what you are after, you can already do those things as a Christian.
As someone else pointed out, it depends on your definition of true, and once you've defined true, you then need to ask, true to whom? You, your community, or God?
I know someone that keeps Orthodox halacha for Kashruth, Shabbat, etc, because he believes that the Oral Tradition is valid, was given by God to Moses, who handed it down through the sages, and so forth. However, he thinks that "Orthodox Judaism," being a response to the Reform movement, stopped its natural course of development, and became fixed in a way it never was before. Therefore he joined the conservative movement because while he at times may think it passes certain halachoth he may not agree with, he believes the movement is more true/authentic than the Orthodox movement. For him, he believes that he is being more true by being "Conservative" and he believes God would agree with him. But the Orthodox world wouldn't agree with him, and if he hadn't have been born Jewish, he would not be recognized as Jewish by the Orthodox movement.
Also, conversions in the Orthodox movement are turning stricter than ever before, in a way that is often inauthentic, or false to the way halacha has normatively been followed for centuries. The Orthodox movement has a nasty habit of saying "The strictest opinion is the correct one! And its always been that way! It's so true, you should never open a book and see how it was done 100 years ago!" i've heard many arguments along the following "Well only the orthodox movement makes sure you learn all the mitzvot which is required! Only the orthodox movement makes sure you don't have alterior motives! Only the Orthodox movement makes sure to disqualify your conversion if you start going off the track later!"
But there's a funny thing about that...
The Talmud, Rambam, and Shulhan Arukh 1) do not demand nor expect a candidate for conversion to learn all the mitzvoth prior to conversion; 2) do not demand nor expect a candidate for conversion to promise to observe all the mitzvoth in specific detail; 3) do not demand an extended period of study before conversion; 4) do not equate conversion with a total acceptance to observe Torah and mitzvoth, but rather see conversion as a way for a non-Jew to become a member of the Jewish people;13 5) do recognize the validity of conversions even when the convert came with ulterior motives, even when the convert was ignorant of basic laws of Judaism; 6) do not allow for the retroactive annulment of a conversion, even when the convert continued to worship idols after converting to Judaism.
And yet the modern "Orthodox Movement" does the opposite of every single one of these points. Replacing the normative halachah/hashkafah with their new opinions, and then attempting to retroactively say that's how things were always done. But the "Orthodox movement" does hold the keys to being Jewish. So any other type of conversion could always cause problems for you later, such as will your kids be considered Jewish for attending Jewish schools, or will you or your partner be allowed to be buried in a Jewish cemetery, etc etc.
So if you want to be recognized as a Jew by every single type of Jewish community, then you HAVE to convert Orthodox. If that isn't a concern, you have many options/movements for you, and some may seem more authentic than others. If you are worried about what God thinks, you may not have to convert at all, please read up on Noachides.
In the end your best bet you should speak to a competent and honest Rabbi who is aware of all the halakhic opinions and can help render a decision that's best for you.
My then girlfriend now wife was wanting to convert, we asked an Orthodox Rabbi that we knew who he would recommend for the conversion. He recommended she convert conservative. Why? Because she is a professional musician, who sings in public, which would be problematic for most converting Orthodox Rabbis to sign off on. And also because the Orthodox Rabbi himself had to convert Orthodox twice because his first conversion was revoked, not because he had done anything, but the Rabbi who had overseen his conversion was disqualified, and so all his prior conversions became revoked.