How To Install Minecraft Mods
At the time of writing, there is no official modding support available for Minecraft (though it is planned for future releases and should be included in the final game.) There are a great many Minecraft mods created by the community and in my opinion, Minecraft would probably have all but died out a long time ago if it weren't for the dedicated and talented modders who have extended the game in a variety of interesting and customizable ways.
Because there is no 'official' modding API, the modding community by and large uses Rigasumi's Mod Loader, which is simply a mod that manages other mods and ensures that they don't conflict with one another. Because this is a matter of community consensus, and not a hard requirement, some mods may not use Mod Loader, others will absolutely require it to work. You may as well familiarize yourself with it, because you're going to be seeing that name an awful lot if you spend any amount of time modding your Minecraft game.
Installation of Minecraft mods is usually fairly easy. You will however, need a program like WinRAR or WinZip. WinRAR has an extensive free trial, and I'd recommend that you start there if you don't want to spend money on modding your Minecraft game.
To mod your game on a Windows machine, press CTRL + R and open the command window. Then type %appdata% into that box and press run. A corresponding window will open and you'll see a file named .minecraft inside it. Open that file, then open the bin file. Once you've opened the bin, you'll see the minecraft.jar. This file is a java application and can only be opened with a program like WinRAR or WinZip.
Open it with one of those programs and you'll see a bunch of folders and an even larger number of .class files. You've arrived at the place where you add mods. Now its time to open the mod file and drag and drop the files inside it into your minecraft.jar. If this is your first time modding the game, you'll need to delete the META INF file before you close the window, or you'll get the infamous Minecraft black screen.
That's it! Modding Minecraft is usually a simple matter of dropping files into the .jar. If this all sounds too daunting for you, TFC's mod manager is a stand alone tool that handles all the behind the scenes action for you, and I'd reccomend that you check it out.