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.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
Let's look, How to win Soul Gems... ^_^
I hope my guide has helped you, this is my first proper articulation, so I may have done several lines. I'm sorry.