Please read this:
https://www.vexforum.com/t/psa-to-new-forum-users-v2/117335
TL;DR: Don’t ask for ideas on the forum
As per how a catapult works, nearly all designs utilize slip gears. I probably can’t explain this very well, but a great resource for getting a basic understanding of most things Vex is the Purdue BLRS Wiki.
https://wiki.purduesigbots.com/hardware/shooting-mechanisms/catapult
Good luck building your catapult!