Lester Crest will give you a mission. But before you begin, make the three characters invest on stocks. For the Multi Target Assassination, make them all invest in Debonaire Cigarettes (LCN), while you can make them all invest in BetaPharmaceuticals (BAWSAQ) for the Hotel Assassination.
For Construction Assassination, invest in Gold Goast (LCN) while in Vice Assassination, they should invest in Fruit (BAWSAQ). Do this before the mission starts because once it does, your investment will be limited to Franklin.
Once you complete a mission, proceed to a safehouse and sleep for three different times to kill time by 18 hours. Once awake, there will be a big return on your stocks. Sell all of it and earn money.
Another way is to assassinate a rival company's leader and take advantage of the stock increase of the company you have invested in. Don't forget to make all three characters to invest in the same stock.
A faster way to make money is to rob businesses. Enter a store and point your gun to the cashier to hold it up and rob money. You can then escape with all the money the clerk will put inside a bag. You can also blow the doors off the back of a bank van to steal all the cash inside.
A tank can also be very useful and you can steal one while you are at Ford Zancudo. Be cautious once inside, there is high military resistance. Use Franklin and a fast vehicle to break into the military base, which you must enter via the west side gate on the map that connects to the main highway. Before you enter, input Franklin's special skills and as long as you can, slow down the time. This will prevent your Wanted level from going to four stars. When you get in, look for a tank and climb to the top of it. Kick the driver out and the tank is yours.
While you're at it, go inside the radio tower building, take a left and walk through the double doors at the hallway. Climb the stairs up to the top, where you will find a mini gun that will immediately be in your inventory regardless if you die. You can buy its ammunition at Ammu-Nation stores.