It's not far fom the city and there are buses that will take you there (or look for a place that will give a tour -transport and gide). I recomend going when its the rainy season, just because everything will be green and beautiful. It's not a huge site, but it is well taken care of. You can just walk around by yourself and wonder at the place. There are pretty nice views from the top of the temples or just about everywhere. You can also look around the museum, but it's not that big and as I remmember they dont have much inside it, but still nice if you are not very knowledgable on the place. And you really can't visit Oaxaca and say you didn't went there.