HELP! I just found out I was adopted but mom doesn't know I know? what to do?

You should let her know. There's no reason to hide it at this point, really there wasn't any reason for her to hide the fact you were adopted at all. Just sit down when she has some free time and talk to her about it. Let her know how you feel, ask any questions you have, etc. Don't worry about hurting her, in the long run this will be good for both of you.