I need to reactivate my IPhone 3g please show me how

I was given a IPhone 3g but it needs to be reactivated and I don't know how to do this can anyone help me please, when I tried to put my music on it, it would only put on 6 songs then it told me it was full, so I checked the capacity and it read 6.7GB so I restored it and now it will not work at all