If you want to gain a 100% completion in the game, you can check it in your stats menu, you'll need to complete all of the missions given to you in the game. This includes the core storyline missions, as well as asset missions, such as the Zero and airstrip. If you have any of these left on your map, finish them all.

However, there are still numerous other requirements for 100%. Below is a checklist check it out.

Assets

Buy all safehouses in the game.
Obtain the Zero RC Shop in San Fierro.
Obtain Wang Auto in San Fierro.
Obtain the Verdant Meadows Airstrip in Verdant Meadows.

Challenges

Complete the BMX Challenge in Glen Park (Los Santos).
Complete the NRG-500 Challenge at the Docks in San Fierro.
Complete the Lowrider Challenge in El Corona (Los Santos).
Complete the Chiliad Challenge on Mt. Chiliad (southwest corner of the map, three races in all).

Courier Missions

Complete all Courier missions for Roboi's Food Mart in Los Santos.
Complete all Courier missions for Hippy Shopper in San Fierro.
Complete all Courier missions for Burger Shot in Las Venturas.

Freight Train

Complete Level 2.

Gyms

Learn new fighting moves at the gyms in Los Santos, San Fierro, and Las Venturas.

Import/Export

Complete Exports for all boards (30 cars total on three boards).

Quarry Missions

Complete missions one through seven.

Race Tournaments

Get 1st Place in all races in Los Santos race tournament (nine races).
Get 1st Place in all races in San Fierro race tournament (six races).
Get 1st Place in all races in Las Venturas Airport race tournament (flying races, six total).
Get 1st Place in all races in Las Venturas Airport Freight Depot race tournament (ground races, four total).

Stadium Events

8-Track
Blood Bowl
Dirt Track
Kickstart

Training Schools

Complete Driving School in San Fierro.
Complete Flight School at the airstrip in Verdant Meadows.
Complete Bike School in Las Venturas.
Complete Boat School in Tierra Robada.
Complete the Ammu-Nation shooting challenge. (Not available in all Ammu-Nations, check the ones in the big cities.

Trucking Missions