You do not need 100% completion per se, but rather you will have to do the following:
Complete story mode - See "Party at the Ranch!"
Collect all Skate Ranch pieces.
Complete Classic Mode - see "Classic Mode 100%"
Find all gaps - This is the hard one. You must hit all gaps, either in story mode, free skate, classic, or a mixture of all 3. There are some gaps that you need to get in story mode, in the skate ranch. This is by far the most time-consuming part of the game. For help with the gaps, try http://thps3.net/THAW/.
For further help, check the notes section
To enter cheats, start from the main menu, then go to 'Options' and 'Cheat Codes'. Enter these:
Perfect Grind: grindXpert
Perfect Manual: 2wheels!
Perfect Skitch: h!tchar!de
Moon Gravity: 2them00n
Full Special Meter: uronfire