Make yourself available. If you want to make friends, you first need to put yourself out there somehow in order to meet people. If you just sit alone, friends might come to you – but that's not likely. If you're still in school, sit somewhere with other people. It doesn't have to be a crowded table, but one with at least two other people.
Remember, friends seldom come knocking on your door while you sit at home playing on your laptop.