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.
This is by far the best free to play MMO out there at the moment. Its well polished, with good sound effects, well detailed tanks and terrain and regular updates and maps.