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.
Two things I use to make money really quickly (other than what the other answers cover) are:

* Burglary missions, and
* Taking Gang Area

**Burglary**

If you park the van relatively near two houses/ apartment doors, you can alternate between them, as the state of a house is reset when you enter another one. 

* Only steal TVs, as this nets you the most cash per item.
* Don't bother sneaking. The TV is in front of you and you get 10 seconds to leave once you've woken them up.
* Jog in, grab TV, leave, drop off TV, go to other door. Rinse and Repeat.

I've gotten upwards of $20k in a 'night' like this

**Gang Area**

Taking over gang area not only nets you a greater passive 'gang' income (next to your house), but you can pick up the money that the Ballas/Vagos drop when you kill them.

This has the added benefit of upping your skills with guns. In the harder areas (more filled colour) the enemies drop SMGs and AK-47s by the last wave - I'm usually maxed out in at least these two guns (possibly machine pistols as well) by the time I leave for San Fierro.

If you have trouble taking over area, I've got a few tips:

* **Start with the lighter-shaded areas** (mostly the industrial areas below Grove Street, but also in the shopping district (away from the residential area) and the beach in the south-west of Los Santos) The enemies will be fewer in number and usually wont get assault weapons (SMG, AK-47) on the third wave.
* **Take enemies from a distance** - if a few of them have reached a range from which they can fire at you, you'll die quickly. Try to kite them as much as possible.
* **Enemies spawn behind you** - Mostly. They always spawn along the footpaths out of your line of sight (behind the camera)   

	+ This includes side streets behind you as well.
	+ They will also tend to stick to footpaths until they are in range of hitting you - use this as an early warning to back away.
* **Collect what you can** at the end of a wave, and face away from where you want them to spawn.  

	+ For most roads I tend to alternate ends to fight on, this way I can run through all the ammo/money and health/armour pickups whilst the enemies begin to spawn behind me.
* **Complete the Ambulance missions** if you feel like you just don't have enough health. It'll take around 30-40 minutes (real-time) but it completely maxes out your health bar, with an added 50% extra. Worth it for most of the game, not just gang area.
	+ Try to grab armour pickups as well. you can also up armour to 150% by completing Police missions, but this isn't as useful as the Ambulance missions.
* **Switch to your other weapon, it's faster than reloading.** (Thanks Sgt. Foley) The game doesn't keep track of magazine status. If you use a gun down to 1 bullet left, and switch out and back, the magazine is reloaded. Take advantage of this. It is actually faster than reloading.