if he's a confident guy with his friends:- just walk to hem and tell hem that you wanna talk to hem in private then tell hem "whats ur name?." then say in cocky and funny way "anyway I noticed that you keep lookin at me, so what's up with that (his name)?" he will smile and the conversation will flow by itself. (TIP: lets say his name was "Dan" when you say his name say it "Danny" just play with the name so you can make an impression). I promise you he won't turn you down even if you were ugly as fuk, the key is by being confident.
if he's not a confident guy: don't bother he ain't worth it.
if you just wanna catch his eye you can: 1- dress hot, 2- stair at hem for a while, 3- raise ur voice while you talking, 4- while you walk crash into hem and say ur sorry and keep walking, 5- and last and best one is just go and talk to hem. but remember, be confident! guys will laugh at ur face if ur not.