How do i loosen up around guys?

ok. i hang out with my friend all the time. we have been friends for more than ten years. im not the type to be social so dont ask me how we became friends in the first place. She sez im fun and funny and she sez that i would have no problem with getn a boyfriend if i just learned to open up a little. But when ever there is a guy around that i like, i freeze up and/or avoid him. plus, i dont think im really that pretty, but i get lots of compliments. I usually blame the fact that i dont have a boyfriend on the belief that there is sumthing wrong with me, but my friend sez im crazy! she's like, "ur great to hang out with, ur funny, ur independant and ur cute, so stop being so anit-social and just have fun". but im havn a bit of trouble. so i guess what i want to know for starters is how do i get a guys attention and keep it? cuz guys seem a bit promiscuous and im uncertain.