The Musee du Louvre started off it's life as a Royal residence in the late 12th century but the building we see today was constructed in phases over several hundred years, from about mid 16th century onward. The two long sides are the Denon and Richelieu wings. The Richelieu wing has actually been a museum open to the public since the Revolution and the Louvre itself had ceased to be the main Royal residence about 100 years before that when Louis XIV decamped to Versailles and took the government with him.
Today it's probably one of the most visited places in Paris and it houses thousands of artworks, artifacts, sculpture, Egyptian and many more types of art, many of which are some of the most famous pieces of art in the world including Leonardo DaVinci's Mona Lisa, the armless Venus de Milo and the Winged Victory. If you plan to go to the Louvre, it's probably best not to attempt to do the whole museum in one visit. There's just too much to see and it covers 650,000 square feet which is a fair old distance to walk! It's far better to take it in small bites of a couple of hours a day over a few days if there is quite a lot that you want to see. Alternatively, you can use a guide book or join a "highlights" tour through the museum. Probably a good idea to choose what you want to see most and make your way there via the map you can pick up at the entrance. The only caveat I would warn here is that the map seemed to me to be a bit confusing because one floor may be more than one level and things didn't seem to be found where they ought to be according to the map.
The museum is closed on Tuesdays for maintenance, and open all other days from 9 a.m. to 6 p.m. Wednesday and Friday it is open later until 10 p.m. and I would recommend going on one of the evening openings. It will be far less crowded if you head in about 6 p.m. (Check the website for the various holiday days of the year that the museum is also closed or closes early) There are other entrances besides the main pyramid entrance but the main entrance is the only one for the late openings. The other entrances are: Porte de Lions which is on the Seine side of the building past the Arch du Carroussel towards the Tuileries Gardens; 99 Rue Rivoli on the other wing of the building; Through the Arc du Carrousel in the courtyard. You can also get to the entrance hall from the Metro Palais-Royal-Musée stop. To get to the museum, many busses, including the city hop on hop off tour busses stop in the courtyard, the Metro as mentioned, stops there and the Batobus has a stop at the Louvre as well. There are disabled entrances and there are lifts and ramps in much of the building.
current fees are 9 euro for an adult which is a full day pass so you can leave and return. If you go to the evening opening after 6 p.m. it's a little cheaper. This does not include access to temporary exhibitions but you can either purchase tickets for those separately or as a combination ticket (13 euros or 11 evenings). There are discounts for seniors and children. You can also buy tickets online and if you have bought the Paris museum pass, you can get in without queuing and that pass would be the thing to have if you plan to return over several days to take more in.
Visiting the major pieces, the Venus de Milo and the Mona Lisa, will likely be crowded any time you go but less so in the evening. We went on a Wednesday night and there might have been a half dozen people looking at the Venus. The Mona Lisa is in a room that has some paintings on the walls but is itself on a free standing wall in the middle of the room with a roped off viewing area. I found it quite off putting, to tell the truth. Very "tourist trap" feeling. The painting is also behind glass so if taking a photo of it, take it at an angle or without a flash which you can do because the room is well lit.
After your visit, you can walk over to see the Arc du Carroussel and walk down towards Place de la Concorde through the Tuileries gardens. There used to be a palace at this end as well but it is long gone. There is still a nice little museum there, well worth a look and far less exhausting than the Louvre itself. The gardens have lots of sculpure to look at and manicured gardens which are very nice in the summer. In Concorde, there is a large observation wheel. The "pods" are free hanging and very low seated but the views are really quite good. They are open to the wind and weather, mind you, so if you've got a problem with heights or if it's windy, it might not be a good idea. There's a canopy over them so you do get shielded from the rain a little bit unless it's windy.
Louvre's English website