How can I start a conversation with the guy I fancy?

Some examples and samples:
Are you in any of my classes @ school?
Do you like cheese?
What's your favorite movie?
Who do you think would be a better President, Obama or Romney?
Do you play football? Baseball? Basketball? Lacrosse?
Hi.
Hey.
What kind of music do you like?
What's up?
What're you doing this weekend?
Which came first in your opinion, the chicken or the egg?
Yo.

Make him a sandwich

if u hear him say anything about something u like, for example a tv-show, movie or video-game u could always start talk to him about that.
then it won't seem like you're desperate :) u just want to talk about the awesome thing he brought up to begin with.
don't think it'll work unless u hang out in the same circles though... 'cuz if u don't it might seem wierd to just jump into a conversation, just sayin'.

Say hi and talk to him like you would normally talk to anyone else.

Just ask him about anything

His favorite movie
His favorite reality TV show
His favorite snack
His favorite electronic device
Favorite game
Favorite subject
If he's in any of ur classes

.....so on