I have experience with going to Dachau on my own. It is so easy to do and much less expensive on your own rather than with one of the "profit added" tours. All of the guides, either from the Memorial or with a commercial operation, have been trained and/or tested by the Memorial, but their own guides take a much more rigorous course. I think most of the commercial guides were originally Memorial guides. A guided tour by one of the Memorial's guides costs 3€.
Here is the Memorial's website with times.
Assuming you are using an inner zone Tageskarte (6€, single, 11,20€, up to 5 people) that day anyway, get a Munich XXL pass (8,10€, single, 14,20, up to 5) instead. That pass will cover both the S-Bahn to Dachau station and the bus from there to the Memorial. Take the S2 S-Bahn, direction (Richtung) Petershausen, to Dachau station. In front of the station are bus slips and the one to Dachau is clearly marked.