As early as your JUNIOR year in high school...
1) Send out game footage, along with individual skills (passing, setting, hitting, serving, etc) to college teams for which you would like to play.
2) Call or e-mail the schools you are interested in and give them a heads-up about you, that you want to play, and where to see you play.
3) Go to the camps offered at the colleges you are interested in playing for and then TALK to the coaching staff and make them aware you would like to play for them.
4) Grades matter!
5) Be persistant.
6) Don't forget, you are "interviewing" them too. Check our their program, coaches, philosophy, etc. to be sure the team and school will be the best fit for you too!
7) Use sites like NCSA to put your information out there for colleges to view...