Easy go on Youtube and Search 'How to Install Mod on Minecraft"
The installation same as minecraft
Example:
Installing Too Many Items
1. go to run and type '%appdata%' and click run.. It'll pop up a folder name Roaming
2. Click on .minecraft folder, then bin
3. Right Click minecraft.jar and Open with WinZip/7zip/WinRar
4. Open the Too Many Item.zip that you downloaded ( here's the link :
http://www.minecraftdl.com/too-many-items-mod/ )
5. Drag all file from Too Many Item.zip to the minecraft.jar.. then delete the META_INF folder
6. And you're done!
Any question about modding ask from me... I would like to help - AO Qing