What class should I choose at the beginning of the game?

Farmer is the best choice since you need to collect and gather resources and build up your base to get stronger later on.

All classes are possibly good choices, although farmer is perhaps the easiest to go with as you passively gain more resources and have a small depot protection to save some resources if you are attacked.

Trader earns more money, Builder makes it easier to upgrade buildings at low levels, the raider is less useful initially unless you are actively playing and want to earn through attacking. It is a riskier strategy but you can earn more if you play a lot and are happy to make enemies.