Question: There are quite some houses and clothes for sale, but I do not have any money. What is the fastest way to get money in GTA San Andreas?

Answer: 
Running taxi missions can get you a small nest egg, which you then want to invest in the horse races - the odds of winning are pretty unrealistic. I seem to remember it being the same odds of success regardless of the odds on the horse. 

Save your game before you start, and then bet the max you can on the longest-shot horse. If your horse wins, save, if not, reload your old save. In a couple of runs at this you'll have enough cash to clear the rest of the game easily.
The best way is in a place called "*Inside Track*" - The horseracing establishments. I started with $4000 and after an hour betting I ended with $5,510,028.