San Fierro is the second of the three main cities in San Andreas. Take some time to get used to your surroundings before you start doing some missions. Note that when you get to San Fierro, you gain access to two courier missions, which will disrupt Big Smoke's drug business. These are not required for 100%, and you will get a call from Cesar on those days the couriers will be available.
On Mondays and Fridays, there will be a Patriot that has $1800 total. Each time you ram into him, $300 packages will be dropped. If the vehicle is destroyed, then so are the money packages. Each time you successively do the mission, there will be gang members that fire at you from the Patriot. The other courier mission is on Wednesdays and Saturdays, and involves stopping a motorbike handler from reaching his destination. You have to kill the courier and then retrieve his backpack for the money. You can puncture the bag if you punch him or fire at him, and the money you get at the end will slowly diminish.