If you decide to take a guided tour, we definitely recommend getting an early morning time slot. You'll beat the crowds and have a much more enjoyable experience. We've included a link which provides many different guided tours depending on what you'd like to see.
Don't forget to visit the Trevi Fountain while you're in Rome. It's much prettier to visit at night when the lights are on. No matter the time of day there are always crowds. *Please note: the fountain is cleaned early morning every Monday and Friday starting at 8am to remove the coins. If you're planning a visit these days, make sure it is later in the afternoon.*