Last year, my husband & I went to the Colosseum (as we were instructed to do) and met a young man outisde setting up tours (as we were told there would be). No reservation was needed and it was a very well organized operation. I don't remember exactly what the cost was but it included admission through the "short" line and fully guided tour. Worked out very well for us.

I've been told you can also book directly with the Colosseum. You can call them at +39 06 39967700 - I'm told English is available.