You are not alone! Many, many kids your age (and adults as well) are shy. And most people are afraid to present in front of a large group of people. It can be scary!
My daughter has always been very shy, and just recently began opening up and feeling more comfortable approaching people. She is in 10th grade, too. But she is still bashful to do things like place orders in restaurants or ask for help from a clerk in a store. It’s getting better, though - I think some of it goes away with age and experience. Sometimes you just have to jump in and do something in order to learn that it’s really not that big a deal after all.
As for making new friends and talking with people, it’s a skill you learn over time, and to some extent you just have to force yourself to take initiative. Often, the other person is just as shy and will be relieved that someone came over to talk to them.
Don’t worry about the blushing. Odds are, no one else even notices, though it seems like a big deal to you. If stuttering is an issue (and is something new, not a speech impediment that would need a speech therapist), just take a deep breath and give yourself a few extra seconds to gather your thoughts. It might seem like an eternity to you, but again, it’s likely no one else will even notice. It’s just nerves. And if you stumble over your words, just smile and start over again. It happens to all of us!