How to active GSM SIM on iphone 4?

Hi there!! This is Sougata From Bangladesh. Hey I just bought an iphone, in this phone, there is no icloud locked/link… and when I check this iphone’s IMEI it says it is unlocked.. so that I can use any GSM SIM.. But when I insert a SIM and connect to itune for activation then itune says that this phone is linked to an icloud account……………….. sign in to active this iphone.. but I don’t know the email and password.. so that I can’t activate my iphone… now how can I use sim on my iPhone??? please help me to solve my problem… thanks