You'll gain experience points for completing jobs on time or early, for having your truck and trailer intact, and parking. As you gain XP, you'll level up, and also gain points which can be used to unlock special skills, which further allow you to make more money per job and gain more XP.
There are six skills you can unlock in the game:
- Hazardous Cargo: Allows you to transport hazardous goods, like explosives, gases, and bio-hazard items.
- Long Distance: Allows you to drive longer distances; expect long-haul trips, like a 14-hour drive from Northern Nevada to Arizona, for example.
- High Value Cargo: Transport loads that have a high value, like diggers and forklifts. These also just look really cool on the back of your truck.
- Fragile Cargo: Transport fragile goods, such as cars, glass, and electronics.
- Just-In-Time Delivery (JIT): These ones are tricky- the company will ask you to delivery the cargo in a shorter time frame than usual. You'll have to drive fast, avoid fuel and rest breaks, and avoid any sort of accidents. However, the pay is quite good for delivering in time.
- Fuel Economy: Decreases your fuel consumption by 5-10%, allowing you to drive for longer distances without fuelling up.
On my first playthrough, I made the mistake of putting too many points into Fuel Economy, but it's pretty much the most useless one.
I recommend unlocking Hazardous Cargo, Long Distance Travel, JIT, High Value Cargo, Fragile Cargo, and Fuel Economy—in that order. You can put one point towards each one in the beginning so you get a mix of them all, but definitely put the most points towards the first three, as they'll be the most beneficial in terms of money and XP as you continue on.