There are six different skills available for you to upgrade each time you level up, by using your skill points. The first skill is the ADR Training, upgrading this skill will let you transport dangerous goods for bigger incomes and you can start off with any class you prefer out of the six classes offered by this skill.
Long Distance - Is the second and one of the most important skills, if you upgrade it often, you will get longer deliveries, more experience points by choosing them and a bigger income.
High Value Cargo - Upgrading this skill, will let you deliver more valuable cargo for a higher income.
Fragile Cargo - This skill is similar to the previous one, upgrading this one will let you deliver fragile cargo for a higher reward. But keep in mind that fragile cargos can get damaged more easily than the rest.
Just-In-Time Delivery - Upgrading this skill will unlock urgent deliveries, giving you less time to deliver the cargo and get higher rewards as long as you do it in time without being late.
Ecodriving - This skill is a really useful one, upgrading it will help you save fuel when driving with a trailer or without one.