Some mods rrequire you to do special things, but this tutorial will tell you how to install the most basic mods. Before we get started, see if the mod you are downloading has instructions, if not, then just use this tutorial, although make sure it also is updated for the version of minecraft you are running. Once that is done, proceed on.

First, get winRAR or 7zip, as you will need them to do anything. http://www.rarlab.com/ After that, download modloader too. http://www.modloader.com/modloader/ And of course, whatever mod you hope to be installing. Open Modloader and the mod you are installing with winRAR, and also navigate to your minecraft bin folder. On a windows like I'm using, you press start, type in %appdata% (No less, no more), and click on roaming, from there you can get to a folder called .minecraft, open that, and you can find your bin from there. With winrar, open your bin, and get into your minecraft.jar, take care not to open minecraft 1.2.4 or 5.jar, as that is the wrong one. Delete a folder called META-INF. Then, copy the class files in modloader into it, along with the class files of your mod. Some mods may require extra things to be put in, I would take care to know that.

Run minecraft now, and you're done!