Watch an advertising video from the main menu to earn 50 gold.
Scoring
When a level begins, focus on one side. The cars will move towards the center at exactly the same rate. Eventually all the cars will be meeting at one point. Try to take out as many of them as possible to earn points before they meet. After shooting the first car, the combo multiplier will increase. Shoot another car before the combo counter disappears to increment it. This will result in more points and coins for each combo chained together.
Upgrades
Start by purchasing the clip size or fire rate upgrade first. Either of those upgrades will help maintain your combos, allowing you to score points and gold easier. The semi-auto upgrade is too expensive at first, at 7,500 gold.