Hey there are lot of organised walking tours in Paris. But I am not sure about free ones. Else I would suggest Google maps could be a great help or get a free map from any travel shop. I took a City bus tour and all you need is to just sit on the bus untill the bus takes a full round of city.
Most European cities have multiple free walking tours i at different points of time everyday starting from popular landmarks. Just pick one after Googling and enjoy the walk. Tip the guide though. Mostly students doing it to supplement their income
Multiple in Paris. It's a huge city so none of the free walking tours do the entire city but just a section of it. i suggest the Notre Dame area with the waling tour as it has amazing istory. You can easily do the rest by yourself. Even though the walking tour is free you have to give a small tip at the end. lets say minimum is 5 euros