How to reactivate iphone 3g?

My mum gave me a iphone 3g it was activated, and it had Iowa 3.1.3 and I was updating it on itunes but it was taking so long to backup. I heard people say its quicker if you restore it and update. I don't have a backup for it. And now it is deactivated. It is on o2 and I put a o2 Sim card in it, and it said Sim card not valid. My dad has a iphone 3g maybe I could restore mine from his backup, or would that not work? Also my o2 Sim is lagged, do I need to get it activated and put it in my iphone or something? I need help, Im stuck.