I think i like this guy, what do i do?

there's this boy in my english cl (he sits behind me) and we talk sometimes and i know his mate(s) coz they're in my tutor and i'm kinda mates with one of them. anyway so i made a joke the other day and now he keeps saying it whenever he pes me in the corridor or at the group we hav to go to. before one of the group meetings it had been raining so i came in soaked and my hair literally in a triangle, he was wet too and when i went to feel his hair (coz he looked drenched) he didn't pull away (but he was being 'stroked' by two other girls aswell). he's really popular and i'm not (to be frank) and i heard that he has a girlfriend but I don't want to break them up. i go all funny whenever he looks at me and i smile at the mere thought of him but i don't want to seem obsessed. what do i do, i kno he doesn't like me but maybe some tips on how to become friends without making it obvious i like him? thank u!