I am a lefty and have always wanted to play the guitar (for more than 20 years), but really struggled as all that was ever around was a right-handed guitar.
This Christmas, I was lucky enough to get a left handed guitar - What a difference!!! I am motivated and loving it! Although I am slowly chugging along, you can make out The House of the Rising Sun, Jolene, Hallelujah, Puff the Magic Dragon and Dream emanating from my fingertips 
Not bad for two weeks!!!
Jokes about keeping rhythm aside, if you are truly left handed and want to play, get a left handed guitar!