Dont just start the conversation by "i like you"
try giving the person your e-mail or your phone number at the end of the convo if you like him or her
never continu to flirt if you dont like the person
be funny and smile
when you are with a shy guy at school u might start the conversation about how hard the pop quiz was and letting him borrow homework then move on to soo what did u do with the time that u should be studyiing ask if he needs help studing and give cell number away