Why am i always nervous?

I know this girl in my history class, I'm 19 and she is 21. She seems cool , I kind of got to know her a bit but not really. i just hate the fact that i get really nervous around her i really can't think of anything to say and if i do i sound scared or shaky. I am myself, when i talk i walked her home i think like twice never got her number or anything but to me she seems awesome, i just need to stop being awkward and build up courage to talk but how, being nervous sucks. I don't even think she'll like me she's older and maybe into better things than i or maybe she likes tall guys idk i kind f short i also hate that most girls like tall guys and I'm 5'4". Anyone got some advice i need some real good confidence i suck when it comes to girls jeez bummer =(