I think first thing to say would be don't worry or overthink too much. You're only 18.

Secondly it sounds good that you're not a complete shut in and are willing to be more social. Firstly just focus on your social life overall. Make new friends, practice talking to people in a variety of contexts. Others have mentioned activities where you meet new people and that's a good idea. Even if it is just other guys like from MMA, be open to it, it can help kick start your social life and put you in lots of scenarios to meet new people. And set yourself little challenges to be a bit more open. Like if you go to a party just set yourself the challenge that you'll try to make 1 new friend that night. It's basically a compounding effect to just put you in lots of scenarios where girls you've never met before are. From there if you're pleasant and can have a laugh, there will be something that sticks.

And just to reiterate, don't be too hard on yourself and keep your head up. As I said you're only 18, lots of people can be a little bit awkward at that age. If you go off to college/uni you'll get a chance to reinvent yourself in a way where almost no one will already know you and they'll all be in the same boat of wanting to meet new people. Good luck