My mom got her citizenship when I was 17. Im now 26. How do I find out that I'm applicable to get my citizenship too. Am I a citizen already through her or I have to file separate now?

There are number of questions that need to be answered before one can give you an accurtae answer.

Firstly, at the time of your mother Citizenship, did you hold a green card and where you on her application for Naturalization?

Assuming you did, and in the absence of anything to the contrary, there is a strong presumption that you are a citizen in law.