Hello! You seem friendly. I'm trying to be more involved in the fandom and wanted to know if anyone has advice! What should I post, what should I say, general stuff like that. Since you have this q&a thing going, I figured it wouldn't hurt to ask. :)
Thank you for this ask. I definitely have advice! I like making friends/acquaintances in fandom spaces, so here is my advice, from my own experience. I think it will be a little predictable, but it still rings true.
For what to post... reblog things! Reblog the work other people make. Reblog fic, reblog art, reblog memes, reblog meta. Add tags. If you see an AU or project you like, check out the OP's tag for it, read through it, send them an ask. Off anon! To be involved with the fandom, be engaged with others in the fandom. Meet them where they are and see what they are sharing. Be excited about and supportive of the things the people around you are doing.
As for what to say... I have been collecting resources (read: I stole these from @/justleaveacommentfest—sorry Skelly) as to how to leave comments on fanfic. These, I think, generally translate.
The gist is this: Specific comments, when possible, which reference a specific detail or two from the post you are reblogging. Say you like their use of colors or their characterization or something. One of those posts has a long list of the types of things you can say, so check those links. But whatever you do, make sure whatever you say is something you mean. Be genuine in your kindness. Do not be backhanded, just point out the things you admire or enjoy.
I have made many, many dear friends through simply being excited about the things they have made. I have also made many friends from people being excited at the things I have made. I have made plenty more acquaintances, and simply passed others like ships in the night. All of those have been very valuable.
Crucially, not everyone you leave tags on or send asks to is going to end up being your friend. I would actually say that most will not. Do not approach this with expectation or entitlement. But even if not all of them end up lasting friendships, most of them are at least going to remember you. You will make their day, maybe their whole week. They will recognize your name with fondness, and they will feel more motivated to keep creating. And you may attract other friends—I have followed many people just because I liked their kind tags on the works of other people.
People like feeling appreciated. They will remember and think fondly on the people who they can tell appreciate their work. They like passion. When you are constantly sharing and showing your appreciation, it will make the people around you happier.
...I also have advice for posting your own work, but then this ask is getting a little long. I do not want to overwhelm you. If this was helpful, you can feel free to send a follow up ask. And if anyone else has advice, feel free to add it!