I need the dumb people's guide on shipping a product sold on ebay.?

i just sold this video game on ebay and i already have the bubble envelope. I have the address, but this is the first time i ever shipped a package through usps. I don't know whether to print a shipping label or take it to the front desk at my local post office. do i just put a regular stamp on the brown envelope or do i go to the post office and they give me some type of scan sticker. I do not have adhesive labels. help please. and please make your answers detailed, for this is my first time shipping anything and i am a bit confused by the whole procedure. thank you!