It is not possible to download NBA 2k13 for free. In order to download NBA 2k13 onto your computer you must buy it and download it from websites that sell games online.
If you are interested in buying NBA 2k13, you may access websites such as store.steampowered.com in order to buy and download the game. NBA 2k13 sells for $29.99. While there may be other options there is no way to download it for free. While downloading the game using a torrent may seem like another possible option, it is important to keep in mind that it's not legal. It is recommended that you buy the game rather than download it illegally.