How to script a crate system like Counter Strike:Global Offensive?

HI I'm looking to make a roleplaying game, but I want to reward players that keep on coming back and to put more content in my game. So I thought about a Random crate opener, I have no Clue how to script it. I try asking on the roblox forums

crate.commonbox
if open 'items' chance 
"items" = The Blizzard,Freezing shield;
Chance The Blizzard (20%)
Chance the Freezing Shield(8%)
FindFirstChild after Spinning for (30)