This achievement is quite easy and will likely be one of the first ones you get in the game. On your map are gas icons. Drive up to the gas station and turn off your engine with "E" and use the confirm button to start pumping gas. Once you're done, you'll unlock the achievement.
Your best bet, if you don't unlock this naturally, is to fire up a quick job and head to the gas station. The employer will pay for your gas bill so it won't deduct from your earnings.