Ty'siPhoneHelp is a YouTube channel that is all about jailbreaking etc. Jailbreak me.com is the only untethered jailbreak for
iOS 4. You need to a tethered jailbreak. You basically install some software from sites like Greenposi0n or Redsn0w. Use his tutorials, he's great.
Once you've jailbroke, go on to Cydia. You need to add this source:
http://repo.insanelyi.com. To do this you need to navigate to the sources page (forgot where, but its the middle button) Manage > Sources > add Source. Once you've hit 'done' search iAppCracker. Hit install, and it should work.
Wanna know how to use it?
Oh and you need a USB cable.