What is the most popular left handed Yamaha acoustic guitar?

Yamaha's most popular left handed acoustic is the APX700II in Natural. This electro acoustic boasts a solid spruce top and mahogany back and sides. It is great as both and entry level guitar and an instrument for more experienced players.

Do I have to play a left handed guitar?

No, not necessarily. Lots of left handed guitarists actually use right handed guitars. The best way to figure out which is best for you is simply to give them both a try. Don't worry if you're a complete beginner. Just holding each guitar in the playing position will be enough to determine your preference!