First of all here is a list of things you have to do to get 100% completion:
-Finish story mode
-Collect all 300 minikits
-Collect all 30 red bricks
-Save all 25 hostages
-Purchase all characters, vehicles, suit upgrades, extras, and information in the Data section
There are many cheats that can help you unlock vehicles, suit upgrades, etc. You can also use cheats to unlock all the extras so you don't have to use a separate guide to collect all the red power bricks. You can find all the cheats here:
http://www.cheatcc.com/xbox360/legobatmancheatscodes.htmlThere are also many walkthroughs on YouTube on the hostage locations, minikit locations, etc. 360AchievementGuides provides good walkthrough videos.
http://www.youtube.com/user/360AchievementGuidesIf you ever need help with the story mode, mario11168 makes good story mode walkthroughs.
http://www.youtube.com/user/mario11168You will have 100% after doing all of this, happy gaming!