Can i change the sim in my iphone 4?

Just got an iphone 4, to activate it It says I need a sim card, i've got my dads one but once i put it in can i take it out, get my sim cut and then put it in, or do i have to wait.

Yes, in the iPhone box that came with the phone there should be a sim eject tool. If you don't know where to look, don't have the box, or are too lazy, etc you may use a paper clip. On the side of the iPhone in the mid plate there's a rectangular outline and a small hole. Put the paper clip or eject tool in the little hole, and the sim card holder with the card in it should pop out. That's when you insert yours.