I think I love my best friend but I also don't know if I'm a lesbian. How can I make sure what is what?

You can try to imagine yourself with different people of different genders in sexual and romantic contexts and be as honest with yourself as you can about how that makes you feel. You can analyze your feelings and sensations, your physical and emotional response to these fantasies will give you a clue of what your orientation is. If fantasies about two (or more!) genders feel good, you're bisexual. If you feel good only about your same gender, you're lesbian, and if what feels good are men, you are straight. I know it's not that simple, but luckily, you have the concrete case of a specific girl that you can focus on in this process. Imagine yourself in these contexts with her. What feels good? The emotional connection? The physical intimacy? All of it? Try to focus on these questions, you'll figure it out!