What shouldn’t I update?
If an update for your device OR for iTunes comes out DO NOT UPDATE.
Let me repeat;
DO NOT UPDATE ITUNES ON YOUR COMPUTER!
DO NOT UPDATE THE FIRMWARE ON YOUR IPHONE/IPOD TOUCH!
Updating iTunes may prevent your device from syncing.
Updating your device will un-jailbreak and re-lock your device! If you are not on AT&T and you update your firmware you will NOT be able to use your iPhone.

What can I update?
You can and should update packages in Cydia, or apps from the apps store.

Can I restore from a backup?
No. Even if you back up the device after it has been jailbroken or unlocked you CAN NOT restore. If you do your device will no longer be jailbroken or unlocked.

What if I accidentally undo the jailbreak or unlock somehow?
If you are comfortable doing it yourself I can guide you through it. Otherwise I would advise you come back to me to have your device set up again.

What is Cydia?
Cydia is a package installer. It allows you to add software that Apple did not approve to your iPhone. You should open Cydia periodically (once a week or so) and allow it to check for updates. If an option to upgrade “Complete” or “Essential” is presented it is best to select “Complete”.
Otherwise, aside from themes, Cydia has some pretty advanced stuff inside that you more than likely don’t need.

What is Winterboard?
This is where you turn themes on and off.

How do I get/use themes?
To get themes, open Cydia, tap the Sections tab, scroll through the list to find entries that look like “Themes (battery)” or “Themes (keyboard)” and then browse through those lists for the theme you want. Once you have found the theme there should be an install button in the top corner.
To turn a theme on after installing it open Winterboard, find your theme, tap it (this will add a check next to it’s name), then press the home button. Your phone will respring, which is kind of like rebooting.

There may be more to this guide in the future, but this should cover the important basics.