How do I know for certain about my sexuality?

There is no rush to label sexuality - as a society, we tend to be overly reliant on labels and putting ourselves in boxes when in reality we all experience attraction differently. Just put your happiness first, and that's all that matters!

Well I am bisexual ...and I have been with a girl ...and I have had major crushes on boys ...so yeah I know :)

You don't. Only you can know if you are bisexual or queer or transgender. Nobody else can decide that for you.

It's difficult to know for sure, because sexuality can be fluid and ever-changing. I'd say time is the best way of telling - it's okay if you decide on one label now and change your mind now, or even decide not to settle with a label! Just go with what makes you feel comfortable!

It varies from person to person. Some people know they are straight, others know they are gay. But some people can't know for sure. It's okay not to have a label for what you are, and its okay to not know.

Your orientation is nothing more than what you feel you are. You can try to imagine yourself with different people and be honest with yourself, listen to your physical, mental, emotional response to these fantasies, listen to your own desires and needs. Take your time, and if you feel like it, get in contact with LGBT people who went through the same and listen to their experiences. Everyone's path is unque and you're the only one who can determine who you are, but support is always good!

I think that sexuality isn't always concrete, and that it can evolve over time. I think it's important to be aware of what you are comfortable with and to set certain boundaries with yourself and others so that you can try and avoid putting yourself in uncomfortable or unsafe situations. I think it is important to be confident with yourself individually, before you decide what your sexuality might be and what that means for you. In general, I think sexuality, just like personality, can be fluid, circumstantial, and have a lot of variables that affect how we feel or come to understand ourselves.