How to backup and replace my contacts on my iphone?

I want to completely reset my iphone 4s. I know how to reset it but I want to be sure that I can get all my contacts back on to it. But I want that to be the only thing that goes back on it. Everything else I have like photos are already backed up to my pc. So how do I do that?

You could write them down on paper.
Sometimes seems, we, as a society, have forgotten that option.