Sorry for your loss man, I know how you feel. A good friend of mine was killed my sophmore year of high school. I didn't see her get shot, but rather hit by a car going 70mph in the middle of the city. It was probably the worst week/month/year of my life.
Make sure that no matter what though, you don't end up blaming yourself, or wondering if there was something you could have done to prevent it, or focus too much on hindsight. I did that a lot myself, and it made things a lot worse than they should have been. Be sure to talk to somebody about it, be open with what you're feeling and don't be afraid to talk about anything. There's nothing wrong about seeing a counselor, that's what their there for. Now isn't a time to be the "tough guy" and bottle up what you're feeling. Even if you don't publicly do it, just make sure that you talk to somebody, be it a counselor, or another close friend, or even your family. Just make sure that you don't try to suppress everything. It's one of the worst things you can do.
Best of luck to you, I sincerely hope things get better.