You can earn additional cash for becoming a taxi driver. To begin, you have to steal a taxi cab (hold key for entering the car. Remember that you can also stop the taxi riding next to you). There are two options of getting the job. You can press L3 to get a client or wait from the drive proposition from the central. You accept proposition also with L3.
The rules of taxi driving is fairly easy. You ride to the particular place, client enters the car and then you take him where he wants to. The faster you make, the more tip you get but you have to be careful, because crazy drive and accidents may frighten him and then he may decide to leave the cab without paying. You get about 100 dollars for a mile plus tip which is about 30-60 dollars. When you finish one drive, you can take another one at once or wait.
If you want to complete an achievement with buying a company and making a private drive, you have to buy Down Town Cab Co. and then wait for a proposition. It's not a standard drive, so making a standard drive won't give you an achievement.