Dear Mr./Ms. Bossname,
I have recently received the exciting news that I have been accepted into (program) in (location), starting on (date). Unfortunately, this means that I will be traveling abroad for the next (amount of time). This opportunity is very important to me, and so I'm very sorry to say that I must resign, effective (date).
I have greatly enjoyed my time at this company, and I hope that before I leave I can help ease the way for my replacement. I will be out of touch during my travels, but when I return home I would love to visit and catch up with everyone.
Thank you in advance for your understanding.