If you have the newest OS 7 you cannot get back the activation lock without contacting the seller to get the old password. Once if you can accomplish that you can delete it and create your own iCloud acct.
I hope you bought it from a reputable source, so that you can contact them & have them put in the passcode and wipe the phone or you can just return it. Otherwise you're out of luck!
This is a security function set in place when "Find My iPhone" is activated...you will need to contact the previous owner so they can deactivate the feature...otherwise there is nothing you can do yourself.