I am a gentile, however I am seriously considering to convert.
I would like to do some of the Shabbat traditions just to get a feel of the celebration (can you call it that?) like light the candle and say the prayer afterwards. I don't know yet whether I am "religious enough" to convert and am hoping for some guidance in the prayer.
Is it okay for me to do that even if I don't yet fully understand all aspects of it? Thanks in advance.
Regarding your sidebar about conversion: Decisions about conversion need to be made with the guidance of a competent rabbi who knows you. If you are serious about converting, and you don't already know a rabbi, I implore you to get in touch with a rabbi in your area, or as close to your area as possible. Relationships build over time, but first contact must be made.
As for the question itself, there are a couple of things to consider.
Shabbath (as I spell it on this site), or Shabbos or Shabbat, as most people pronounce it, is a special day set aside by G-d for the Children of Israel (aka today as the Jews) to connect spiritually to Him and to one another. Once a week we declare our belief that G-d created the world and remember that He took us out of slavery in Egypt. In fact, during this declaration, some have a custom to stand as witnesses stand (in Jewish law) when testifying (as explained in paragraph E3a here).
Lighting candles is an important ritual done before the start of Shabbath. One of its primary purposes is to create peace in the home. It is true, in many ways, that candlelight emits what many consider a peaceful glow. In addition to that, in the days before electricity, especially in agrarian societies, one does one's work until shortly before sunset, at which point the only light one has is from a lamp of some sort, so one eats supper and turns in for the night. However, if one is deliberately staying up past nightfall for a festive meal and songs, having candlelight is essential, for preventing injury if nothing else. On top of all of the practical reasons, it is also said that the light of the candles elevates the spiritual level of the home and brings in a spiritual peacefulness.
Taking into account the above, the blessing on the candles, which is "Blessed are You...who has commanded us regarding the lighting of the candles for Shabbath" seems to be out of place for someone who is not Jewish and therefore not obligated in observing Shabbath.
However, I'm not a rabbi, nor am I an expert in this area. I can see a reason for a prospective convert to light candles without the blessing, in order to get a flavor of Shabbath (and get used to performing the ritual).
It was pointed out to me by Isaac Moses in the comments below that there is a chance you are asking about the common practice of reciting a personal prayer and a general prayer for health and wellbeing for one's household. I have to repeat my refrain that I am not a rabbi, nor am I an expert in this area. But I don't see anything wrong with thanking G-d for one's blessings or asking for the general welfare of one's family at any time, provided that you do not use the formula "in the merit of our matriarchs Sarah, Rebecca, Rachel, and Leah," or inclusionary language such as "for us and all of Israel", for the simple reason that you are, at this point, not a member of the Jewish People and the Jewish matriarchs are not your matriarchs, though they may serve as wonderful role models of piety for you.
You should have a very serious conversation with a rabbi about this subject and about your thoughts on conversion and whether it may or may not be for you.
For practical advice you need to consult a rabbi, but here is some general information.
Certain prayers/blessings, including the one said after lighting Shabbat candles, contain the phrase "who has commanded us". Gentiles haven't been commanded, so (a) saying it isn't accurate for you, and (b) you might be taking God's name in vain by saying it. (What exactly the Noachide prohibition on this is a separate question, but I mention this because at least some gentile religions also have this prohibition. If you are a member of such a religion and aren't yet sure you're leaving, you might care about that regardless of what Judaism has to say about it.)
But you aren't looking to practice exactly as an observant Jew does; you're exploring. There's nothing wrong with lighting candles and having a nice family dinner on Friday night, and if you want to use this time to focus more on your relationship with God, that's great! Instead of saying the standard candle blessing, you could say something like "thank you God for this special time", or even thank God for specific things that have gone well in the past week or that you're anticipating in the coming days. (Try not to make specific, mundane requests; Shabbat is about stepping out of ordinary time, and anyway, shouldn't God get to rest on Shabbat too?)
There is a special prayer said by some women after lighting candles that you could adapt. (You probably don't want to use it exactly as-is.) The text, from Aish HaTorah (h/t Isaac Moses):
May it be Your will, Lord my God and God of my fathers, to be gracious to me (and to my spouse, children, parents) and to all my family; grant us and all Israel good and long life; remember us for good and blessing; consider us for salvation and compassion; bless us with great blessings; make our household complete, crowning our home with the feeling of Your Divine Presence dwelling among us.
Make me worthy to raise learned children and grandchildren, who are wise and understanding, who love and fear God, people of truth, holy and attached to God, who will dazzle the world with Torah and goodness and service of God. Please hear our prayers, in the merit of our matriarchs Sarah, Rebecca, Rachel and Leah, and ensure that the glow of our lives will never be dimmed. Show us the glow of Your face and we will be saved. Amen.
To be clear: you aren't "doing Shabbat" by doing this, because Shabbat is a gift for the Jews, but you are setting aside some "God time" from the rest of your week, which is an important part of Shabbat. You probably don't want to call it "Shabbat" or "sabbath", though; it'll just confuse people.
If you do decide to explore conversion -- and not everybody who starts the process ends up converting; it really is exploration at that point and not commitment -- then the rabbi you work with will guide you in taking on practices. Most likely he will have you start saying the "who has commanded us" blessings before you are officially a Jew. But that's for your rabbi to decide; every case is different.
