To get season rewards in Tribes of Midgard, you need to complete worlds by either abandoning them, winning them or escaping using a Bifrost.
Using either of these three methods will level up your season rewards bar, and the amount that it levels the season rewards bar will depend on how well you perform in these worlds.
The main factors are: How many days you survive, how much XP you earn, and how many jotunns you killed. So if you want to farm rewards, the best thing to do is to make sure to earn a lot of XP by clearing camps and slaying the various monsters in the game.
Also, try to survive to around day ten as this will give you the best amount of season rewards XP. You should also try to slay at least three Jotuns, as slaying these giant foes will give you the most out of all three options.
That said, if you have trouble with Jotuns, it probably means your power level is a bit low, so you will need to raise it. You can raise your power level by leveling up and unlocking skills and by getting better gear in the game.
Check out some more Tribes of Midgard guides at Pro Game Guides!