Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
I used to play this game a lot, only do the tasks if you're gonna just play for fun and not for the gyms. The tasks are not good compared to other games and how much they give for the first task.