this is the 2nd time ive decided to go with o2 for a contract and just like last time im gettin fucked off already

i get 750 mins free and i get 750 text for £35 a month yet somehow my first bill come thru for like a hundred quid. i did think yea, ive used it alot but....
what pisses me off is i can never seem to work out wen i get my free minutes again and free messages. its crazy. im scarred to use the bloody fone now not wanting a big bill and unsure if ive used all my mins and texts up.