Finding Mods
Mojang, "Minecraft's" developer, does not have an official repository of game mods, so you have to locate mods on fan sites and forums. The two most popular sites hosting "Minecraft" mods as of publication are Minecraft Forums and Planet Minecraft (see Resources). Search for the mod you want, and then download the mod file to your computer directly from these sites.
Opening "Minecraft" Files
To install "Minecraft" mods, open them in a third-party compression program such as WinRAR or 7-Zip. Leaving that window open, press the "Windows" key, type "%appdata%" (without quotes), and then press "Enter." Open the ".minecraft" folder, open "versions," and then open the folder for the version you want to mod. Right-click the file bearing only the version's number -- not the file with the .json extension -- and choose to open it with your compression program. If you have not done this process before, you may need to click "Open with..." and locate the program on your PC.
Installing Mods
In the "Minecraft" file open in your compression program, you see a list of all the game's current data. Select the "META-INF" folder, and then press "Delete" to remove it. Without this step, the game will not launch with mods. Drag all the files from the mod file's compression window into the "Minecraft" file's window to copy them into the game. If the program warns you about overwriting existing files, click "OK." You can always re-download a clean copy of the game to remove your mods later. Some mods also need files placed in other locations, and include their own instructions to guide you through installation. After copying the files, close all the open windows and launch the game as usual.
Server Mods
Most "Minecraft" mods only work in the single player game. If you run a server, using CraftBukkit is the easiest way to install mods for all your users. CraftBukkit replaces the default "Minecraft" server program entirely. You can mod CraftBukkit through plugins without needing to alter any files -- simply place the plugin files in the "plugins" directory of CraftBukkit and restart the server. Regular mods designed for single player "Minecraft" will not work with CraftBukkit.
Warning
"Minecraft" mods only work on the specific version of "Minecraft" they were created for, usually noted on the mod download page. Installing mods into the wrong version of the game will stop the game from working.