1. Dinner is going to be at least €20 per night
2. Download a map of the metro system on your smartphone
3. The shortest lines for climbing the eiffel tower is late at night (before 11)
4. The shortest lines for Notre Dame are during the morning hours
5. Don't take out your phone and flaunt it around in Monmartre
6. The Louvre is free on Friday nights after 6
7. All museums are free in France on the first Sunday of the month
8. Make sure you know when the metro closes at night so you don't get stranded or have to walk several km in the dark.
9. Say "bonjour" every time you walk into a business and make eye contact with a staff member
10. The service will be slower. That doesn't mean the service is bad. The pace of life is different.
11. Don't expect that you can grab a quick coffee at a Parisian café. You'll be sitting there for at least 45 minutes.
12. Anytime you say you want coffee, they will give you an espresso. If you want an American styled coffee, ask for "café allongé"
13. If you're at a restaurant and you're antsy about getting your check, observe other tables to see whether or not you pay at the counter or they bring it to you. Never shout out that you want the check. If you're in a bit of a rush (Most people who go to a restaurant aren't) politely raise your hand when the waiter comes by and ask for "l'addition." (A.k.a the check).
14. Don't even try getting lunch after 3pm or dinner before 7pm. Most places are only open to serve wine, beer, or coffee.
15. A lot of people speak English but not all are fluent. It's best not to expect them to speak English anyway. Learn a few polite French phrases and chances are if they hear your accent or see that you are struggling, they will speak to you in English the best they can.
16. When you're waiting for a train in the metro, stand at the side of the doors so the descending passengers can get off first, otherwise it's kind of rude.
This are all things I have learned from first-hand experience. Paris is beautiful but it's best to remember that even if you're a tourist, the people there are working and going on with their lives, kind of like New York. Money won't by you everything there, but politeness might.