Iphone 3gs 02 now putting vodafone sim in please help!?!?!?

My brother use to have iphone 3gs but he's got the new iphone 4s! he said i could have he's old iphone as my phones rubbish! we put the sim card in and the pass code for my sim card and everything was fine but it's not coming up with the network i'm on and it's only showing 1 signal bar. He was on 02 and i'm on vodafone so we wonded if that's why! could someone help slove the problem
(if any cost is included into what i need to do then please mention and price) thank you!