First, complete the game to unlock the "Cheats" menu. Unlock the following cheat functions by performing the corresponding tasks in Time Trials:

CHEAT FUNCTION - HOW TO UNLOCK
Full Armor - Start with full body armor. Does not replenish (1 XP).
Tactical Knife - Start with a Tactical Knife (2 XP).
Infinite Ammo - All weapons have unlimited ammo reserves (3 XP).
SB-Y90 - Starts with a SB-Y90 gun equipped (4 XP).
Universal Silencer - Gunshots and ricochets do not alert guards (4 XP).
Proximity Charges - Start with Proximity Charges (4 XP).
Silent Steps - Footsteps no longer alert guards (4 XP).
.50 DLX - Start with the .50 DLX gun (4 XP).
Invisible to Cameras - Become invisible to all cameras (4 XP).
JM7 Rocket - Start with the JM7 Rocket (4 XP).
Golden Gun - Start with the Golden Gun. kills enemies with a single bullet. (5 XP).
Vanisher - When you defeat an enemy, they will disintegrate (6 XP).