Use the cover system effectively. In intense shootouts, you can also loot weapons and ammo from downed enemies to stay "strapped" without spending cash.
Complete 30 "Most Wanted" missions and 20 "Vigilante" missions found in police vehicle computers. Quick References & Resources GTA 4 Guide - IGN grand theft auto IV
To achieve "Key to the City" (100% completion), you must finish more than just the main story: Use the cover system effectively
Complete all main missions and specific side tasks like Little Jacob’s drug deliveries and Brucie’s exotic exports. grand theft auto IV
It is recommended to play the original game first before diving into the DLCs ( The Lost and Damned and The Ballad of Gay Tony ) to avoid plot spoilers.