Theres always glitches, but if your new to the game I wouldnt just yet as it will kill your enthusiam to do any part of the game afterwards as it may seem pointless (brought the t shirt )... just a heads up.
if you have a friend the car dupe glitches which involve another person are usually the most efficient if you own the same facility. Also worth checking reddit /gtaglitches thread as the dude Leo makes well written posts which can be easier to follow than some of the youtubers out there who spend forever repeating themselves
Import Export is generally the way to go to make the best profit for your time. Obviously after a while it does get kinda boring doing the same over and over. I mix it with the Gunrunning supply stuff, VIP works only, sometimes Crates with some friends or chill people in the session.
1st method. Solo method -Buy full supplies for your bunker and MC businesses (if you own any) -Do Import/Export while your supplies run out. -After and hour or two, go check your businesses and buy supplies if necessary. You will see your bank account grows technically immediately.
2nd method. Non-solo method (rage method) -Buy full supplies for your bunker and MC businesses (if you own any) -Host a Pac Standard Heist and pray experienced players join. In the finale just take the bikes-no shooting strategy and you will escape with pretty much the whole cut every single time. -Once you're done with the finale you'll have a 1 hour cool down before you can host Pac Std again. You can join another person's heist or do some I/E missions.
Instead of Pac Std you can host Doomsday Heist Act III which pays out even better, but since it is relatively new many players are not very familiar with the missions and they die more often, but you can try. Act II doesn't pay as well as Pac Std or Act III but it's very easy to complete.
I like method 2 because although low level players sometimes join and ruin the setup missions, they keep it interesting. Also this method is the best way to know experienced players who can invite you to their own finales. You will start making a group of friends who can help each other.