How to you mod a digital copy of a game off steam?

[Red Rocket]

Like seriously, everyone else seems to understand this, but not me.

I bought the game off Steam, and I can't find any files for the game, but I want to mod. Can someone help me step-by-step how exactly you do it, cause I don't know the first thing about this stuff (laugh if you may), but I still a bit stupid...

[Andrew]

Copied and updated from the NBA 2K12 FAQ, so credits to JaoSming for saving me some typing:

Where is my main NBA 2K13 folder?
If you have the Steam version of the game you can find your main NBA 2K13 folder here:
C:\Program Files\Steam\steamapps\common\nba 2k13

The default DVD version of the game installs to;
C:\Program Files\2K Sports\NBA 2K13

Where is my NBA 2K12 Saves folder?
For XP/Vista users goto the Start Menu and click Run.... then type in %appdata% with the % signs. Then click 2K Sports, then NBA 2K13. That is your saves folder.

For Windows 7 users you can type %appdata% in your Start Menu's search bar and press enter. It will take you to your Roaming folder and then you can click 2K Sports and NBA 2K13.