The key to using the Metro system successfully is knowing that you always look for the number of the line you need, then the Terminus or end station on that line. The terminus determines your direction of travel and the platform you need to be on. This is unlike the London Underground system, where the lines have their own names with directions (Central line, westbound) and the New York Subway, where lines are indicated by letters or numbers (Line 2, uptown). Something I learnt on my first trip to Paris when I didn’t know this so got rather lost until I got my bearings !
Therefore, if you are travelling towards say the Arc de Triomphe from Sacre Coeur, using Line 2, you would follow the signs for Ligne 2 Porte Dauphine to guide you in the right direction. There are large Metro maps posted in every station lobby and on each platform, and it is also handy to have a pocket map to consult when above ground.
You never have to wait long for a Metro train. There are 303 metro stations and the trains come every 3 minutes on average. You may have to wait longer during a weekend evening, but this will rarely be for more than 5 minutes. Because the metro system is dense and the stations are close together, the time between each station is roughly only 90 seconds.
Unlike the London Underground system, the trains on each Metro line shuttle from one end to the other, there are no branches where a train may split off on another line half way along and take you to a different terminus station than the one you expected to go to.