What do I do with my low self esteem? Why do guy stares at me?

It is really frustrating bc I have low self esteem. I am not sure if it me that is seeing things or what? I used to have a crush on a guy who I found out has a gf but he is always nice to me. His pupil always get large when I talk to him. I have a guy in class that always stare and if I move, he will stare at me like he is paying attention. I just feel like I can't really read guys. Advice? I think I am giving up on this courtship/looking for a guy...