Instructions
Steal a low rider and go to Loco Low Co. in San Fierro. In order to unlock this garage, you will need to pass the mission "Cesar Vialpando" for the character Sweet. It is one of the earliest missions in the game. Low rider models are Blade, Broadway, Remington, Savanna, Slamvan, Tahoma, Tornado and Voodoo.
Steal a street racer and go to the garage at Wang Autos in San Fierro. In order to unlock this garage, you need to have purchased Wang Autos, finished the driving school challenges, and passed all of the missions for Cesar at the garage in Doherty. Street racer models are Elegy, Flash, Jester, Stratum, Sultan and Uranus.
Steal any other car and head for a TransFender location in Los Santos, San Fierro, or Las Venturas for general modifications. The Los Santos garage is unlocked along with the Loco Low Co. garage when you beat the mission for Sweet. The San Fierro and Las Venturas garages are available as soon as you can access the cities.