One thing's for sure, there will be a line.  You can wait in the main line by the pyramid with the hundreds of other tourists, but typically the line from the underground mall is significantly shorter.  Just enter by the staircase on either side of the Arc de Triomphe du Carrousel and you'll find yourself in a small mall with a Starbucks and Apple Store.  The entry to the Louvre museum is near the inverse pyramid which is a popular photo-op site. There is a kiosk to buy museum passes inside the mall, but if you just want a Louvre ticket you can buy it inside the museum once you get through security.  After security the main concourse of the Louvre has multiple ticketing counters but unless you need to buy a special discounted ticket or don't have a credit card with a microchip, you don't need to waste your time in that line.  Right beside the counter are several self-service kiosks where you can pick your language and buy adult (full-priced) tickets with a swipe of your card. Because the lines for the counter are so busy it can be hard to tell you don't need to stand there, so just head for the yellow kiosk and you'll be set.

There are several different wings of the Louvre but if you're like me you're going to hit all of them so you can really start anywhere. Make sure you don't bring bulky bookbags because you will be asked to check them before entering the museum wings. Also be prepared for some warm temperatures in the summer months. In August there was no sign of A/C and the sheer number of people crowded into some areas made for very uncomfortable rooms. There aren't any water fountains but you can buy bottled water at the various Louvre cafes in each wing. Yes it's expensive. A big bottle was around 5 euro. You'll buy it anyway because all of that learning and pondering and pushing your way through crowds of cell phones to see your favorite paintings is hard work.

A Quick Virtual Tour: