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.
The concept of World of Tanks, it's to take a point or eliminate, the other players. I have been playing it for a while, and it is a fun game.