As I'm a high school student, I was wondering if it's okay for me to ask questions on ELL about grammar and so forth although English is my first language. If so, which topics can I ask questions about?
You should try to ask questions that would be of interest to those who are learning the language as a second language.
We're not here to help you with your homework, but if you can couch your question in a way such that the learning community at large will benefit from the question and its eventual answers, then you can ask it here.
That said, since you are a native speaker, I'd expect your questions would be written with great care and display exemplary grammar.
As for what topics are allowed, do you have any specific questions that aren't answered in the site's Help Center? There are links there that explain what topics are allowed, and what kinds of questions you should avoid asking.
Incidentally, I'm a native speaker, and I've asked a couple questions on ELL. You can have a look at those if you'd like.