1. It's completely normal to struggle at making friends, no matter how old you are.
2. It's important to learn to be comfortable by yourself.
3. In order to make friends, you'll have to start trying new things (but you don't necessarily have to leave your house).
If you try a new hobby and you find you don't like it (or you don't like the community), that's okay! Move on and try something else. The important thing is to keep trying stuff until you find the right thing for you.
4. Eventually, though, you will have to step outside your comfort zone (but just a little!).
"But they might say no!" Yeah, they might. And that's okay! Not everyone has to be your friend, and you only want to make friends with people who want to hang out with you. So just accept it and ask someone else who seems neat. But more people will accept your offer than you expect.
5. For a while at least, you'll be the one taking initiative to ask people to do stuff with you.
Well, for one, other people don't know that you're on a quest to make new friends. And two, while the ultimate goal is to make some close friends, at first your goal will simply be to get more comfortable with inviting people to do stuff with you. The more often you do this, the easier it will be. It will become important later, I promise.
6. Not every friend you make has to be a friend forever.
This might seem like a failure, but it's not. In the process of finding some good friends, you're also learning more about yourself and what you really want from a close friendship. People can come into your life for a short time and still have an important impact.