Fulfill your lifelong dreams of becoming a multimillionaire! In this game, at least.
Method #1: Inside Track Betting
There are two Inside Track horse betting shops in the state of SA. They're open to you from the start of the game. There's one in Los Santos, and one in Red County, specifically in the Montgomery area. I recommend the one in Montgomery, because it's northeast of the Catalina's Hideout safehouse (unlocked after mission First Base).
What you want to do is save, go to Inside Track, and bet a huge sum of money on the teal-coloured horse (this will give you 12x your bet if you win). If you lose, load a save. If you win, save the game. Rinse and repeat.
Method #2: Side Missions
There are an assortment of side missions in SA that give you money, but most of them give so little money it's embarrassing! The best ones to do are the Firefighter missions and the Import/Export missions. In Firefighter, get a firetruck and extinguish cars. You get more and more money for each level you complete, and it's extremely easy to do.
In Import/Export, find one of the vehicles on the list at Easter Basin Docks. Bring it to the drop off point on the ship. Use the crane to pull it up if you want. Firefighter is available from the start of the game, but Import/Export is unlocked after the Customs Fast Track mission.