From WikiCheats

Gameplay

Each movie of the classic trilogy is broken up into 6 levels. As you play through Story Mode, you will venture through the multiple areas of each level in hopes of reaching the end to complete it. To get to the end of any level you will need to solve puzzles, build, break, repair and dig things up as well as fight your way through hordes of Lego baddies.

Studs

Studs are the small round single Lego pieces that will act as the currency for the game. Collecting enough of these will allow you to purchase different playable characters and a variety of cheats. These also pertain attaining the "True Adventurer" status. Every level has a different amount of Studs to be collected in a single play-through to achieve "True Adventurer." You can keep track by watching the bar in the top middle of the screen.

Artifact Pieces & Parcels

Collecting and completing artifacts will get you a ton of extra Studs. Finding and mailing Parcels back to the college will unlock cheats that you can purchase from the Mail Room. Every level in the game has 10 Artifacts Pieces hidden through out the various areas of a level. Each level is also home to one Parcel. Know that where there is a Parcel, there must be a Mailbox near by. You have to mail the Parcel in order to unlock the cheat.