How to delete/reset my iphone's data completely?

I want to get the Samsung Galaxy so I want to sell my iPhone 4S. Therefore, I want to delete all of my infos completely, that includes email account, apps, everything, but how do I do it? And once I do wipe it, will someone able to restore my old info? 10 points for best, serious answer.

Go though the settings and do a factory reset. Either that, or do it from iTunes.

The only way that someone could recover your data would be to take the phone apart and do some jiggery-pokery with the memory. Most do not have that level of ability and those who do are not likely to destroy an iPhone just to get your music and contacts list.