Help: how to code this

I have a form for a user to fill up so that i could collect their data (FB user, full name, date purchased, proof of payment, mode of payment) when they click the submit button the data will be saved to a collection. I managed to do that part but I want the data to check on another collection and look for their FB name in that collection I have created of my FB group members list who purchased my online class. Once the data matched on both data collections it goes to a page where it automatically download the recipe file.
I also want a success message and error message to show up when the submit button is clicked. Error message should show up when they have not filled up all of the elements in the form.

Am not a coder so can anybody help me?

What you are describing is not trivial and will be a challenge for beginner coders. You might want to consider checking out the WixArena - it’s a hub where you can look for Wix Code (and other) experts for hire.

Good luck,