To install a Minecraft mod:
Open the ZIP file of your downloaded mod.
Hold the Windows key and press R on your keyboard at the same time to open Run
In the Run window, type %appdata%
In appdata, go to .minecraft
Open the bin folder
Right click on minecraft.jar and open with WinRAR or whatever program you use
Delete META-INF from minecraft.jar
Drag the files from your downloaded ZIP folder into minecraft.jar
Close the ZIP and jar
Run Minecraft and enjoy your mod
You can find mods at
Minecraft Forum - News, Forums, Mods, and Texture Packs