I can't speak to 18+ clubs since that's really not my scene but there are a lot of ways you can get to know more people and feel connected to the community.
For example you can check out some of the youth programs at the LGBT center. http://www.sfcenter.org/programs/youthprograms/youthprogram.php
They have a bunch of programs tailored to youth specifically. You'll likely have a nice, safe experience with others who are dealing with some of the very same questions you pose here.
And since it's coming up, you might wander down to the Castro Street Fair - and do so especially if you've got friends to go with since it could be a little overwhelming if you're flying solo.
Much luck :>