Im in High School and I find it very difficult to make friends because I am very shy. I do not know what to say to people when they talk to me. Any tips or advice ? Thanks

Well, I'm in my 30's now, and honestly,I have never had your problem... but I can remember kids that did.
Smile. Just be you. Seriously, sounds dumb, but everyone, even the loud people in high school are insecure and are just trying to fit in.
Are there any clubs you are interested in? A sports team? Debate? See what kind of clubs you can get involved in.

Its progress, not perfection, and just remember, high school is a minor bleep in the grand scheme of things, even though it may not feel that way now.
Good luck.