Ok, the single most important thing is:
Always start greeting the person you are about to talk to, in French. A simple "Bonjour, excusez-moi .." ( Hello, excuse me ..) is enough.
Seems like some of my compatriots will not answer at all or notice you until you start with that. ( Yes, Parisians can be pretty cringe worthy if you don't 'do it right' - but past this simple step, and if the person has the time they will gratefully help you ).
The full phrase would be "bonjour, excusez moi, pouvez-vous m'aider? - parlez-vous anglais?"
"Hello, excuse me, could you help me? - do you speak english?"
And of course ned with "merci" or "merci beaucoup" ( thank you/ thank you very much)
Key words or pieces of phrases can be enough to go by with, we won't fault a tourist that go with a mix of both language
, but really the best thing would be to either invest in a phrase book or look up a specific web-site - there should be plenty of them, I think some will even have audio. Try youtube too.
ETA: the rest of etiquette is mostly the same as the US or the UK, use of common sense and observation of the 'locals' can be usefull.
Trying not to be 'that tourist' too - respect the fact that while (general) you is a tourist here, people around are working, living here, they might be busy, want to enjoy the quiet.
I guess they'll be in Paris? It can and will get crowded around touristic spots, and the metro is packed in the morning (I'd say from 7.30 to 9 ish) and afternoon (5ish to 7) depending on the ligne/stop. No need to fuss, best to keep it cool and read up on the ticket/fare system beforehand so as to not blockup the turnstiles.