1) If you arrive in Beauvais airport and you need to find a way to go to Paris city center you can get Bus tickets from here. Because Ryan air which is most of the time cheaper will land only in this airport. Most of the people suggest other airports which is near to Paris but if you can spend one extra hour you can land in Beauvais and come to city center by bus. Because it (flight + bus) may be cheaper than some flight options.

2) Paris city metro is little bit complicated but it covers almost all the areas. It may be cheaper to get Paris 10 tickets options which will give you 10 tickets at once at a discounted price.

http://parisbytrain.com/paris-metro-tickets/

3) Get a metro map and correctly identify zones and lanes. Because there are some connecting lanes which can be confusing a bit. And also make sure you are not going through an exit when changing lanes or metros, Because once you do you ticket become void and have to use another ticket. Some routes combine rail and metro and may be cumbersome to find them. So to make things easier i opted only to use metro even though the ticket is valid for both to make my journeys hassle free.

3) You should visit paris by night which is more beautiful than the day time. I am posting few pics below and you can decide :)

4) Louvre is an amazing place to visit. But it takes time to explore since it is massive. Plan to spend at least  5 to 6 hours of the day there. Make sure you check the timetable as they closes the Museum on time

http://www.louvre.fr/en/hours-admission