Auschwitz only allows their own guides to conduct tours within the site. You need to go to their website and sign up for a tour ahead of time. (It is possible to go into the camp on your own, but only very early or very late in the day, unless things have changed since we were there.) We hired a private driver (not a guide) to get us to the camp and he was helpful in getting us past the long line and into the camp (with our pre-reservation.) Once there, he waited in the parking area as we took the tour led by an Auschwitz-licensed guide. She was excellent and, I think because they carefully choose, train and supervise these guides, that is the norm. She was very dedicated to the memory of what happened there.
There are also many companies in Krakow that organize bus trips to Auschwitz and these trips include the guided tour there. Again, the bus company does not guide the tour itself, but puts you into the hands of a licensed guide. You can find these companies online. You can also just take a regular bus there - I have heard this is cheap and easy. If you do this, be sure to have your tickets for the camp tour- they often sell-out. (Do not take a train. It leaves you off quite far from the camp.)
The Auschwitz tour guide took us through the first camp (Auschwitz), then gave us a little break, then took us on a shuttle to the second camp (Birkenau) and gave us some more touring. She left us there to look around on our own and take the shuttle back when we were ready. You can actually walk between the two camps, but Poland was having an extreme heat wave (almost 100 degrees) so we were grateful for the transportation.
Rick Steve’s guidebook on Warsaw, Gdansk and Krakow has extensive info on visiting Auschwitz, including a self-guided tour. Also look carefully at the info on the Auschwitz website.