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.
There should be an option to pin a thread in order to help new users find the answers they are looking for and to avoid asking similar questions that were tackled before. Only the mods can pin a thread perhaps?