I need to restore my iphone 3gs?

[Dakota C]

I need to restore my iphone and reset it because i have no room. It used to be my dads old phone but he got a new one and gave me this one it still says ***** iphone in tunes.....and whenever i download an app it automatically goes to his phone so if i restore mine i wanna make sure nothing happens to his because hes a ceo of a big company and he has some pretty important contacts so i need help quick pleasee!!!! 10pts