All you have to do is "back up" your iPhone in iTunes -- when you plug your iPhone into your computer it should automatically sync, displaying "Sync In Progress" on your iPhone's screen.
Then depending on what jailbreak tutorial you use, you should have pretty exact instructions on how to jailbreak your iPhone successfully and then use the files from the "back up" to restore your iPhone.
The tutorial I used from from iphonehacks.com
Good luck and enjoy!
PS -- Once your phone is jailbroken, download qTweeter from Cydia...BEST app I've downloaded thus far