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: 
If you start at the downtown hospital, head right, then turn right at the first road, on your left-hand side you'll find an outdoor "mall" or concourse. In here, you can hide from the police helicopter, and cars cannot get at you. There's also a soda machine that you can use to heal yourself.

Run around in circles here, taking out everyone and all cops; you'll get a pile of handguns, and money from all the citizens wandering around, and if you're careful, you'll never get wasted.
You can kill drug dealers for a quick influx of cash - They drop anything up to $2k. Note however that (despite cleaning up the streets) you actually lose a point in respect for each dealer you kill. This doesn't add up to much, you can kill enemy gang members to make up for it, but just be aware.

The dealers that I know of are:

* White guys wearing white hoodies (He is mostly found around Grove Street).
* A Black guy with checked shirts. He is difficult to find though, as he looks very much like civilians.
* Another Black guy wearing a beanie and black t-shirt.

If you stand near them, they will say something like "What'cha need?", which you can respond positively or negatively. They're usually armed with a pistol, so if you plan on taking them out, do so quickly.