An excellent topic. I find the bus to be a far nicer way of getting around the city, unless I'm in a hurry.

No stairs to climb, no turnstiles to deal with (important if you're carrying packages), the seats are far more comfortable, it's far nicer waiting for a bus outside than for a train in the bowels of the Metro, and, in my experience at least, the foul odor experiences for buses are way fewer than for the Metro.

As BvH suggests, get a good guide or map (you can get an inexpensive folding map that shows bus routes), and you're on your way.