Sexual orientation is not always a simple matter, especially if one is beginning to discover who they are attracted to as a sexual partner. Many people feel comfortable with either male or female partners and that is fine.
Just allow yourself the time and space to be yourself and you will find yourself along the way. You also mention another point; sometimes you feel like a guy and others more like a girl… this can be a manifestation of a few different things and I suggest to you that if you need to investigate and feel guided in this process, you can totally consult a psychologist or even a peer counselor at your school or even a local youth LGBT org.
It has been my personal experience that sharing your feelings with a trusted friend or loved one usually helps a lot in better understanding yourself, so this is also something you should look into.
No matter who you turn to, please understand that you are just fine as a person and that it’s ok to have the feelings you are feeling at present.