USPS lets you create shipping labels online. You'll need to set up an account, but that's fairly easy. To make a label, you just need an address and you'll need to know the weight of your package, but they calculate everything from there. Also, you're going to have to choose a type of shipping, but I don't think you need to pay for the fastest service, also beware that there are options for using their provided boxes, don't use those. When you print the label, tape it to the box (or, for advanced users, buy the USPS sticky-back printer-friendly labels). You can just leave the package by your mailbox and the mail carrier will take it with em (to be sure, maybe leave a sign the first couple times you do this) or you can take it to the post office.