Play games
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.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
This game has a lot of knowledge to consume, there is so much customisation, so much abbilities, Warframes (the characters) and much more. Heck you could even make your own world, make your own ship, have a pet and perks and much more. Its astounding to see this much in a video game its just so much and I love it.