iCrazyTeddy has posted two videos one of which show you how to earn unlimited money while the other details what you need to do to get an adder for free in Grand Theft Auto: Online.
In Pot Shot, simply take the van and return it to Trevor's trailer for a sizable reward in around three minutes. Mixed with Coke, meanwhile, you need to be level 45 to start the mission or you can have one of your friends ask you to join. You'll need to blow up four cement trucks, two of which are on the construction site and another two which will spawn elsewhere on the map.
Finally, in Coveted, you need to wait for the helicopter to land, kill the pilot before picking up a container and bringing it to the final location for $15,000.
To get the free Adder, head to the Bugatti spawn location in Los Santos and smash the window to get in. You won't be able to park the car in your garage because it's stolen so you'll need to be playing with someone else to watch over the car and make sure it doesn't vanish.
Then you'll need to enter your garage and enter a car you don't mind losing then open your phone and find a quick job, once you've done that back out of the random job and you should see a glitch motion which will spawn the Adder in your garage. This will be easier if you already have a Bugatti but one will spawn on the map eventually.