Those of us who write letters every year have written lots of letters for kids like you. Don’t worry about it; we’ve traveled this road before.
Your teacher will ask for a 'brag sheet" of sorts, with info to help them craft a more complete letter. That can help a lot, since it can give them the info that less shy kids would have already given them.
Being shy will NOT stand between you and college.
Come up with teachers in classes where you did well. Relax and leave the rest up to the teachers.