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.
Personally, I think the best "Fortnite" type game on Roblox has to be Island Royale or Strucid. They have the best building mechanics and overall best structured game. Check it out!