Start the game, press Resume and press the tilde key to open the console. 
Press Tab to see all commands

Command           Effect
--------------------------------------------------------------------------
G_IgnoreLocks   - Unlocks everything (All cars, tracks, weapons, upgrades,
                  rims, stickers, events, cups).
G_Godlike       - Enables/Disables the Godlike mode (visual damage only).
G_Money [Money] - Change your cash value.
G_Score [Value] - Change your score.
G_RubberBanding - Enables/Disables rubber banding for the AI.
G_AutoReset     - Enables/Disables the auto-reset of the players.
                  (Free Run Mode)

Notes: 
* All commands are case sensitive
* To see all unlocks, go back to the main menu and press Resume
* To unlock everything in Quick Race, open the console in the Quick Race 
   menu and type G_IgnoreLocks.