Regular Mini-Kit Basics
Each level has 10 Mini-Kits, and most of them are inaccessible when you first play a level on story mode. Completing a level this way will unlock free-play mode, and under this mode, you'll be able to use a greater range of characters. Once you've unlocked or purchased at least one Astromech droid, High Jumper, Child, Bounty Hunter, Imperial trooper or officer, Sith and Grappler, you'll have access to all areas while on foot. Unlocking or purchasing a TIE fighter and either a Snowship or a Republic Gunship will give you access to all areas in flying free-play levels.
Tips on Finding Regular Mini-Kits
Destroy everything possible. Mini-Kits are often hidden behind walls or obstacles that can be destroyed, and some Mini-Kits don't appear until you have either defeated a certain number of enemies in a room, smashed a certain number of items or used the Force on a certain number of objects. Don't bother trying to collect every Mini-Kit in story mode; wait until free-play mode to try to find all 10. There's no timer in free-play mode, so take your time and examine everything carefully.
Blue Mini-Kit Basics
Once you complete a level in story mode, you'll unlock both free-play and challenge modes. In challenge mode, you'll have a 10-minute time limit to find 10 blue Mini-Kits. These will be in different locations than the regular Mini-Kit, so you'll have to find them all again. It may take a few attempts to locate all 10 Mini-Kits, so it's best to tackle a few of the shorter levels first to get the hang of finding them under a time limit.
Tips on Finding Blue Mini-Kits
Blue Mini-Kits are always visible without needing to destroy anything or use the Force but are often hidden behind corners or in locations that are difficult to access. If you've unlocked and purchased one of the "Ghost" Jedi characters, you can get through levels without being disturbed by enemies, as they won't be able to see you. This will greatly increase the time you have to look in hidden corners. Droids will also be ignored, but they move very slowly. The Mini-Kits will make a jingling noise as you approach, which will help pinpoint their position.