Xim,
I'm 21 and in college myself, so I understand where you're coming from...
I've said it a million times and I'll say it again, just do your thing, be confident, and don't smother the girl with attention.
If it's a casual thing, don't call the girl and txt her everyday. There is an easy way to let a girl know she's a fuck buddy or not a fuck buddy--and that's that you only contact her when you want to have sex. At MOST, if I haven't called exclusively for sex because the girl has cranked about it, i'll call them ONCE during the middle of the week for a 5 minute check-in, that's it. "Oh, I'm surprised you're calling me, I didn't think that was your style" they'll say. They think they're special cause I'm actually giving them a mid-week call and they like me MORE for it...(lol)...
Some girls get PO'd by this, but others love it. And honestly, the ones who need maintenance in between are not worth it for you anyways, eventually problems will develope. Girls feel dirty when they know everytime your name illuminates their phone display on a Thursday night at 1am that they are getting booty called, and the ones who get a rush from this are the ones you're going to have fun with.
So really the best way to say that "This is casual" is to not say much of anything. Needy guys are the least attractive thing EVER. Attention in the right way and at the right times is key--even I still mess that up sometimes and end up having to scrap the project and start over--but in the end take it all as a learning experience, be confident in yourself, and the rest will just follow.
DO NOT think about and analyze things so much. Most of the girls who are going to fling with you are not going to be the sharpest tools in the shed, and the whole cerebral braniac thing is a big turn off, I should know, cause that's how I am too. You just gotta pretend you're cool AND dumb
Above all, enjoy yourself, and have fun.