We saw a bullfight this time last year in Madrid. Your hotel can probably help you get tickets, but we found that our hotel charged a surcharge. There are ticket offices located around the city and you can get them there with no surcharge. I'm sorry but I can't give you a specific location. Our hotel pointed us in the right direction. Also, try to buy them a few days in advance. We almost didn't get tickets because we waited till 2 days before to buy them.
The arena is round, so I don't think it matters a whole lot where you sit. The seat prices are based on "in the sun" and in the shade". Shady seats are more expensive. We paid about $15 each for seats that on the edge of the sun/shade border. We were in the sun for about 20 minutes, then the sun shifted enough that we were in the shade. We were very glad...there's little air movement in the arena and you're squashed in with a lot of people on bench type seating. It would've been extremely hot if we'd sat in the sun the whole time.

Have fun! It's an interesting experience, not only from the actual bullfight aspect but also from seeing how passionate the locals are about it.