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.
E-100 is a promotional german tank it's earned only by 100 players but i think it should become available for every player as a standard or a premium vehicle.