Bonjour et bonne idée de venir à Berlin, ça vaut le coup
This being said, remember that Berlin's public transportation system may appear excellent compared to that of other German towns, but in no way not to Paris. The distance between underground stations is much bigger, and there are wide areas of the town where there is no underground at all. As a comparison, Berlin's U-Bahn network has a total length of 146 km and has 173 stations, while Paris' Métro is 218 km with 302 stations, and all that in a much smaller area:
That's one aspect. The other one is the fact that Berlin was widely destroyed during the war and big parts of the town are just not worth seeing
So be aware that while in Paris walking from Notre-Dame to the Champs-Elysées, for example, is a matter of something like one or two hours and you will be able to see a lot of things on your way (Châtelet, Louvre, Jardin des Tuileries, Place de la Concorde, Champs-Élysées), in Berlin there are only a few routes worth walking (which in my opinion is the best way to see any town). On the other hand, a bike would be a good alternative. Berlin has a an excellent network of biking lanes, and there are plenty of rent-a-bike offers.
Not sure what you're into of course, but I think the best would be to find a place near "Unter den Linden". You'd be in the heart of the town, near Friedrichstraße for (expensive) shopping and Museumsinsel for museums, Reichstag and Brandenburger Tor for historical sight-seeing, Kreuzberg and Friedrichshain pour les coins "branchés".