Grand Theft Auto San Andreas?

Iv'e just bin told that i don't have to do a single one of Zero's missions to complete it. But its the only mission available to do? So how can i continue in the game? Theres another mission where i have to stop someone on a bike from getting out of the country or something (thats cesears mission) and when i fail it says "you stole �0 from the courier" is that the mission i have to do to carry on with gang stuff?