Here is one of the most viewed "introduction to Tanya" videos on the internet, by one of the worlds most popular Tanya teachers*, I recommend watching it to decide if Tanya is for you.
How Tanya is the story of You
There are several ways to sum up what Tanya is, but from the above lecture: people would come to the Alter Rebbe and ask him what appears to be a simple question, how can I turn my mitzva performance into a service? The Rebbe answered based on the pasuk which says that it is natural for a Jew to serve Hashem with his heart, i.e. with love and fear. People were effectively asking
"why does it not feel natural to love and fear Hashem, and bring that
devotion into my service, despite the fact the Torah claims that these
things should be natural?"
So the Rebbe wrote Tanya, to help people get to the source of the issue. If a person is finding it hard to love Hashem, there are 2 potential causes. Either the person doesn't understand himself well enough, or he doesn't understand Hashem well enough, as if both of these are in place the love indeed will be natural.
So the Tanya contains two main sections, Likkutei Amarim, which is also called "Sefer Shel Benoni", which aims at introducing a Jew to his soul, and his true potential, and what such a soul and potential's mission in this world is. In this section, we also learn lots about Hashem, as well His personal purpose in creating.
The other main section is Shaar HaYichud VeHaEmuna, which is all about Hashem. If a person doesn't love Hashem, it's because the picture one has of Him is an unlovable one, so the Rebbe reintroduces us to Hashem, bringing into down to earth language everything the philosophers and kabbalists debated and ironed out over the preceding 1,000 years about Hashem, how great He is, how close He is, how much He loves us, and as Rabbi Friedman shows many times over his lectures, how much He needs us.
There are two more sections to the Tanya as well, if the above is not enough, that take things even deeper and answer even more amazing questions about Hashem and our souls.
I would recommend learning all of Tanya with some sort of class that connects to you, rather than just trying to read it for yourself. There's so much more to get out of it when it is taught by a loving student of the Chabad school who can bring it all together, as well as fill in gaps and give context and add additional teachings and developments from the 7 generations of Chabad Rebbeim who have discussed it.
* Rabbi Manis Friedman. On Chabad.org, there are a handful of names with lectures on every daily Tanya, and he is one of them. Another I'd recommend is Rabbi Ronnie Fine.