Well, there's no way I can tell you to be sure. I'm a lesbian, but at 19 I was sitll unsure whether I was lesbian or bisexual..or what. I think it's easy to get hung up on labels because it makes it easier to get on in the world.
I think the pressure to self-identify can be terrific. I suppose just think about who you are attracted to; if it's predominantly other females, then I'd say you were a lesbian.
While genuine bisexuality does exist, it's also important to realize that society conditions you to expect to be with a man. I know many, many gay people who first thought they were bisexual. We all changed our minds and so can you!
Sexuality is exciting and confusing; it's important first and foremost to just be comfortable with who you are. The labels will (or won't) come from that; you'll be fine.