Spanish, French, and German are the easy ones because they're the most common in high schools, but see if you can start something that's off the beaten path.
Might I suggest Swahili, Hebrew, or Korean. I personally started a trial version of Rosetta Stone's Japanese class and actually started picking things up in just a few minutes.
Maybe, just maybe, you'll need a new language to read a future story of mine. Better get ready now.