I highly recommend an introductory guided tour given by one of the Louvre official guides. There aren't a lot of these tours in English--perhaps one or two a day-- you'll be lucky if you can snag one. It's a good introduction. They'll give you radio headphones to wear so you can always hear the guide no matter how many people in the room. As stated before, you can go on your own afterwards.
There is some tour info on line on the Louvre website, but you have to really search for it. I couldn't find the link just now. Even the website is huge, but highly interesting, with focus lessons, themmatic trails, as stated above, etc.
For a Louvre guided tour, you buy your ticket in a separate area once you are under the pyramid. Go where it says Accueil des Groupes. The ticket counter is in there and that is where your tour group meets up for the visit. Oh, and there's a toilet back there.
I spent 6 months in classes for adults at the Louvre, and there are still rooms I haven't seen.