Don't use the taxis parked directly outside the entrances at the Eurostar terminus without checking the fare. There's a reason there isn't a queue. They will try to charge ten times (or more) the official rate. They have all the scam rates printed to make it look official. Also watch out for ring droppers, look it up, they tried to catch me. And, yes, pickpockets everywhere, often using someone to divert your attention such as a 'student' taking a survey, 'deaf' girl with a petition.
The metro is a great way to get around and I bought tickets in tens. It saved time and money.
If you're doing a lot of walking in the city, buy the little red book of street maps. It's easier to carry and doesn't mark you out as a tourist, and therefore a target, like a fold out map
Having said all that, Paris is a wonderful city and we love it.