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.
Right now there is an event where you can collect eggs from games in Roblox. Definately try out some of the games and try and get as many eggs as you can from the egg hunt. There are 49 missing eggs in different games. https://www.roblox.com/egghunt .