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.
I enjoyed reading the lord of the rings novel. I used to play games based on this novel like middle earth shadow of mordor and war. Although I don't know about TLOTR war, maybe I will try.