The assignments officer will contact you your senior year around Jan-Mar and present you with several available locations to choose from. There is no way to forcast prior to your senior year what might be open.
Also remember supply and demand...lots of people may all want certain locations (Hawaii, Italy, Japan, Korea) - not everyone can go. Small locations may only have a few spots opening up the summer you are graduating (sometimes none at all)...if a spot doesn't open up, it's not an option.