Minecraft Mods-How do you install?

First, check to see if the mod you're trying to install needs Modloader and / or Forge API to run, as most mods do. You can go on the Minecraft Forums (www.minecraftforums.net) to search for these. They have very detailed guides for installing.

I only know about installing mods on Windows, so if you're a Mac user, you should go to this link:
http://minecraft.gamepedia.com/...lling_Mods

Windows ME/95/98/XP:
1.Click start.
2.Click Run…
3.Type %appdata%/.minecraft/bin
4.Open minecraft.jar (It may appear only as minecraft).
5.Open the folder using an archive software (the most popular are 7zip and WinRAR). To do this, right-click minecraft.jar and hover to the Open With menu.
6.Delete the META-INF folder (IMPORTANT; Minecraft can't run mods if you don't delete this)
7.Copy the mod's class files into this folder. Exit the folder once you have copied the class files.

Windows Vista / 7:
1.Click the Start Icon.
2.Go to C:/Users/(username)/Appdata/roaming/.min…
3.Open bin and you will see minecraft.jar (It may appear only as minecraft).
4.Open the folder using an archive software (the most popular are 7zip and WinRAR). To do this, right-click on minecraft.jar and hover to the Open With menu.
5.Copy the class files into this folder. Exit the folder once you have copied the class files.

Please refer to the Minecraft Wiki for future reference.
minecraftwiki.net

Hope this helps.