To return to the basic, default icon all you need to do is click on the app or file, press Command-I to bring up the information window, click on the small icon up in the top left hand corner, and press delete.
When you want to customise individual icons yourself, the easiest way is to...
1. Select the icon you want to change your folder, app or file to.
2. Choose "Get Info" from the File Menu, or click Command-I.
3. Click the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type Command-C).
5. Select your folder, app or file on the desktop and "Get Info" for it.
6. Click the icon in the upper left of the info window.
7. Select "Paste" (Command-V) from the Edit Menu.
For system wide changes and to change icons like the Trash and Finder, you might like to take a look at
CandyBar.