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.
If you are new to roblox I would recommend first customizing your avatar. After go to the front game page on the top left it should say games then click on any game that looks interesting to you. Bye!