During highschool, I lived in the country, so I wasn't able to be super social outside of highschool. I kept busy with sports during the school year, and I've had a job in the summers since I was 14, but I still wasted quite a bit of time in my room. Now that I'm in college, I live in an apartment that people come over for games (one of my roommates is a game freak) every single night. Having a dozen people in here, just hanging out, playing video games, board games, and talking isn't unusual.
Anyways, sometimes its difficult, yea. What changed it for me is I decided to do things other people are afraid to do. In the summers I build and drive demo derby cars. I moved across the country to go to college. I took a cross country road trip this summer through the rockys, and multiple other road trips in the past year.
Don't be afraid to go out and just do stuff you've always wanted. Make plans to do stuff with friends. Start a game night with your friends. We have regular Dungeons and Dragons gamenights, and it's always a hoot (as long as you have a good Dungeon Master) or something like that, that can take up a lot of time without getting monotonous.