Basically i got someone elses iphone unlocked and now i am using it on 02. However when i try to get onto the internet without a wifi connection it says, "you are not subscribed to a cellular data plan" or something like that...
If your current O2 tariff doesn't have any data allowance, then you either won't be able to use the internet, or they will charge extortionate prices per MB.
The only solution is to buy a data add-on, or change tariffs to one that has internet included.