1st, go to the USPS website:
http://www.usps.com/
You can calculate foreign or domestic postage for various types of service, so you'll know how much to charge your buyer. I have mostly used Airmail Letter Post or Global Priority Mail.
You'll need some Customs forms, "PS Form 2976", which you can pick up for free at any Post office. You fill out this form in advance, and I could be wrong, but I have always thought it was necessary to then stand in line at the P.O. to turn in the package and let them attach the customs form. I fill out the form accurately, and don't mark it as a gift" unless it is.
It's more hassle than domestic shipping, but not too bad. The worst part is if there is a long line at the P.O.