Instructions
Head to Gamespot and check out their complete Grand Theft Auto IV game guide. It's got all the information you need to finish the game's main missions and become king of the Liberty City streets (see Resources below).
Cruise over to IGN's GTA IV page to get your hands on another game guide. Although one's usually enough to get the job done, having a second one handy gives you another reference in case you get lost or can't figure out one's instructions (see Resources below).
Read through Gamespot's list of cell phone cheats. There are about two dozen different numbers you can punch into Niko's phone, and each one changes the game in a unique way.
Browse through the different user-created GTA IV walkthroughs and strategy guides posted at the bottom of Gamespot's page. These often go into more detail than the official game guides and can help you tackle specific missions.
Check out the IGN GTA IV Game Wiki, where users can upload their own hints, tips and cheats and share them with other gamers. This is a great place to get obscure information and unorthodox strategies (see Resources below).