How to erase Android user generated files (such as pictures/videos)? Factory default?

I have a Samsung galaxy s3 that I want to sell. There is some personal images + videos that I want to remove first.

Also Gmail and viber is attached to this device, these accounts can no longer be linked to the phone. The simcard is locked (but I can get some access through the usb connected to the computer), how do I return this thing to "factory default" or as close as possible?

I want the files to be permanently deleted. Not possible to recover.

Any help and suggestions for tools and where to look for files is appreciated. Thanks.

 

Thanks for all the links. I will read them and see how it works out.

 

I think that did the trick. Thanks.