Make a Simple Mod of a Game

A game mod is a modification of a game that changes its properties. This how-to shows you how to customize your game with little or no programming.

Steps

Locate the game folder, which is usually in "C:\Program Files", and find common file types to replace.
Use Notepad or another text editor to modify an .ini file (e.g. gunAmmo(26) to gunAmmo(255)). Some .dat * .cfg files can also be modified, but don't try it if you see a bunch of funny symbols
Use Paint or another graphics editor to modify image files. These include files with the extensions .bmp (bitmap), .gif (Graphics Interchange Format), .png (portable network graphics), and .jpeg (Joint Photographic Experts Group), among others. Usually these image files are textures to be mapped to certain surfaces or pictures to be used on the game's intro or setup screens.
Replace .wav (wave), .mp3 (mpeg3), and .ogg (ogg vorbis) sound and music files with your own. These are audio files that are used for music tracks or sound effects inside a game.

For fun you can replace the .wav files with your own recordings, so for an explosion you just blow into a mic for half a second, or gargle a very small amount of spit.
.ini files, though they are very useful for game modding, are rarely seen, because programs these days now store their configuration settings in the Windows Registry.
Replace audio files with appropriate sounds. This keeps the game and the actions you performed relative to the sound itself. For example, it wouldn't be very realistic to hear anything other than an explosion when a missile strikes a building.
Use google to find mods for your game. There are usually whole websites devoted to modding games. They also (sometimes) have special tools and forums.
Learn a bit about programming, variables, and hexadecimal. This will help in editing files.

Never touch the Windows Registry, where most games these days store their configuration files, unless you absolutely know what you're doing.
Don't mess around with files that have weird symbols or gibberish. These files, if edited, can ruin the entire game.
Don't try to mod files with weird extensions, it could ruin your game.
Be careful modding online games cause some moderators think you're hacking and will kick you off the game, or get a VAC ban if you hack on steam!

Text Editor for .ini files. Notepad is fine.
A basic knowledge of programming helps.
Picture Program for .bmp, .png, .gif, and other picture file formats. Paint works well.A good photo and image editor that's free is Gimp and another, more featured but paid, is Photoshop.
Program such as Windows Media Player for ripping music.