Download redsn0w here: http://blog.iPhone-Dev.org

This jailbreak will ONLY work on the following devices:

iPhone 3GS
iPhone 4
iPhone 4 (CDMA)
iPad 1
iPod touch 3G
iPod touch 4G

More info on this jailbreak: http://blog.iphone-dev.org/post/14857834236/untethered-holidays

Steps:

1. Download redsn0w and extract the program from the .ZIP folder.
2a. Windows users: Right-click redsn0w.exe and “Run as administrator”
2b. Mac users: Open redsn0w.app
3. In redsn0w, click the “Jailbreak” button.
4. Plug your device into your computer and turn it OFF.
5. Click “Next”
6. Enter DFU mode by following the instructions.
7. Ensure that “Install Cydia” is checked. Click “Next”
8. Wait for redsn0w to complete the jailbreak process.

Done! Cydia is now available on your homescreen.