Everyone knows what happened here but nothing can prepare you for what you'll see and feel here. Whether to go or not is a very personal decision, I am very glad I went and on a selfish level it really helped me put the trivial problems in my life into perspective.
Practicalities: If you're staying in Krakow I would strongly recommend NOT taking an organised tour from there, all the ones I looked at only gave you 3-4 hours or so and most didn't include a guided tour of the Auschwitz I camp. It's not a place to rush so would suggest you leave your options open in case you want to spend more time there than an organised tour would allow. There are regular official guided tours in English (and other languages) for 40PLN which you can buy once there, which include both sites plus transport to and from Birkenau with the guide. The guide was excellent and spoke excellent English, it took about 4 hours total but does not cover all the exhibitions in the Auschwitz I camp, there wouldn't be enough time, but you are free to go back in after the return trip from Birkenau to see these. I spent another 2 hours seeing these.
Travelling from Krakow is easy, just look for buses to Osweicim Museum from the main bus station, takes about 90 mins. There are regular buses back to Krakow into the evening, and there are a couple of restaurants/snack bars across the road if you're up to eating before going back.