Before you travel to Prague it is a good idea to download and print a map of Prague. Prague Airport Transfers offers a map of Prague city centre as a free download. You can also obtain this map with a pocket guide from your driver, if you book an airport transfer from the airport. This Prague map shows you some of the main areas of interest, such as the Old Town Square, Charles Bridge, Wenceslas Square and Prague Castle. This map also includes a map of the Prague Public Transport system, which will pretty much get you anywhere in the city that you want to go. It is a very up-to-date and easy-to-use map, but if you need a larger-scale map with Prague suburbs, you can download a free map from Czech Tourism. Their map is considerably larger and is devoted to sport and relaxation activities.
For those who do not need extra planning in advance, you might be just fine getting simple maps from hotel reception desks or from tourist information centres. The quality of those maps might vary, and will sometimes include quite significant amounts of advertising rather than useful information you are looking for. But they are free, which is good.
Lastly, there are some good maps for sale or included in guidebooks, but they are not really necessary for short-term visitors. Most visitors do not venture outside the city centre areas, and free maps will do just the same job as paid maps.