There are many ways to make money; legal and illegal and I will go over a few different ways more commonly used here.
There are a variety of crops you can farm with a specific field for each type of crop. The legal crops currently are:
You would need to buy the seeds for these crops from the farmers market; currently located at New Haven. For each crop there is a specific location for it to be grown at, for this example I will be using Cotton, the field for Cotton is found near the Lakeside Correctional Facility. To plant; you just scroll on your mouse and hit 'Plant Seed', they can be placed quite close to each other, allowing you to fill the field quite efficiently if you do it in a regular way. These crops take around [-] minutes to grow after planting you will be able to harvest them by using T+Windows/Start Key which will give you an amount of the crop, depending on your skill at farming. The crop will then be stored in your virtual inventory, where you will be able to return to the farmers market to sell your crop, and restock on the seeds.
The illegal crops you can farm are:
These seeds can be bought at one of the two drug dealers, and then planted at the farm location for the crop. As it is illegal, if caught, you could receive a long jail sentence; so even though there is more profit, there is a higher risk to do this crop. To further maximise your profit, you can process your opium/cannabis at a processor (turning it into heroine/marijuana) which will require you to purchase a license, and a small fee each time to process it. This will increase the money made when selling the product to the drug dealer; however it will lead to longer, and more dangerous journey's.
Another method being used more recently; is fishing.
Fishing requires you to buy your own boat; with prices being found on the Vehicle Dealerships page. You will take your boat away from the shoreline, where you will find the option (scroll wheeling) to drop your net and begin fishing. You can then continue this to fill your virtual inventory, and then onto a truck or any other vehicle you brought. These fish must then be sold to the 'Fish Market' which is located in the centre of Lakeside, near the garage.
This is similar to fishing, however you must be more geared up. The gear you will need includes a wetsuit, SDAR (underwater gun), diving goggles and a rebreather. You will need to sail to one of three poaching areas, where you will dismount your boat, take out your SDAR and dive under the sea. When you are deep enough, turtles will appear, this is when you need to shoot the turtle; then harvest the meat and take it to one of the traders. However with turtle poaching there is an element of risk, it is illegal and the police force will actively try and prevent you from poaching the turtles.
This is another relatively popular method of making money, with the advantage being it is legal with the hunting license. The only need for hunting, is a hunting rifle and a scope. All you need to do is drive to the hunting area (preferably with an off-road capable vehicle) and look for something to hunt (such as goats). You then take the shot, and harvest the meat off the animal to take to the butchers at Old Bedford where you can sell it. Each round for the hunting rifle (CZ550) costs $4500, so make each shot count. However with hunting; you can do it with a pistol which will make it cheaper overall.