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.
Will the current minigames stay for a long time or do they refresh them every month or something like that? I am getting pretty bored of the current minigames...