Welcome Pen!
I believe the minimum age here is 13, so you’re good! No worries, don’t feel like you need to rush in! We have people of all ages here. You’re right, there’s a huge variety of subjects. I don’t think it’s possible to know them all, so just pick whatever calls to you most!
I suggest reading some introductory articles on the main site, and taking a quiz like in this topic - to get some ideas on what to start on:
We’re all chill and friendly here, if you have any difficulties, don’t be afraid to ask!
Cheers. 