As per usual with every response to introductions, welcome to the forums.
I don't exactly have any tips on the game itself and I haven't played much in a while, but I do know a couple things about the forums in this "Other" category.
General Discussions are for conversations unrelated to osu!, and are usually just for anything general or life-related, whatever. Basic forum.
Off-Topic is hell. Never go there. Never. You'll catch the two strains of diseases involved in the OT epidemic, called Trashiputus-B and Trashiputus-M. You'll be sucked into a new world of shitposts you can never escape from. However, bridge the gap, and you'll find the two oasis subforums within Off-Topic, those being Forum Games and Surveys. Both of those are fine and are safe.
Then there's Introductions, where we're at now. Self-explanatory.
Music Hall is a good place if you either make music or are a music enthusiast. Even if you really just have a song that you happen to really like or have a topic related to music that you have in mind, you're still free to post there. A good place to find new music if you dig around, as you get to have an idea of the music other people like.
Otaku Culture, a place for movies, anime, manga, or whatever entertainment that revolves around... well, otaku culture.
Video Games is self-explanatory.
Art is a good place if you're an artist or you really want to get something cool-looking, as there are graphic designers and whatnot that offer a chance for you to get a cool little graphic for your profile page, avatar, or signature.
Uh... I'll be on my way, then. But it'll be nice to see you around. Good luck.