This is a story related achievement, you will get this while playing through the main story.
*** Spoiler - click to reveal ***Complete The Green Sabre mission.
Previous mission: Reuniting the Families
Next mission: Badland
Main mission progress: about 14%
Location: Los Santos
Start location: Los Santos, Sweet's house (S logo on the map)
Reward: Access to San Fierro and the countryside
Video walkthrough: GTA Series Videos
Buy some weapon, ammo and a body armor before start the mission, but not too much, because you will lose all of your weapons after the mission. In the first part of the mission you have to meet with Cesar. Only driving, no fighting, very easy. In the last part you have to save Sweet before he dies. When you arriving to the destination, hit the enemies by car, then park in the red marker. After this eliminate the enemy waves until the police arrive.