Depends on what your interests are.
I'm very inexperienced with birthday parties as well, but wanted to have one this year. I invited my three closest friends, and we just hung out and played video games all day, and went out to eat for dinner. Was a good time, and didn't need to be that elaborate or anything.
Watching movies (at home or at a theater), playing board/card games, browsing YouTube or something together, going out for a walk, going shopping, going out to eat, sharing stories... There are several things you could do to pass the time and have fun.
Also, having snacks and drinks ready for everyone to munch on is good.