My top five favourite apps for getting around Paris (and they are all free as well!).
I use this app every single day. The best function is the route planner. You just plug in your start and end points (the address or metro station) and Bob's your uncle, it'll give you the exact directions (metro, train, buses, trams - whatever you select although I tend to stick to the metro because there is less chance of getting lost that way) of how to get there. It will also estimate the time needed to arrive at your destination (I always add on an extra 5-10 minutes for delays/wrong turns).
To double check, rule of thumb is to allocate 2 minutes for every station and 5 minutes for each station change to be on the safe side.
Best of all, the app is available in English and there are always three options to choose from: fastest, least amount of walking and least amount changes.