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.
Which team do you follow, and why? I gotta go with NaVi, been following them for many years and it's nice to see them win the recent Katowice tournament and return to the top of the game :)