How to show/hide fields on user input?

How to show/hide fields on user input?

Hi I'm trying to build a new form for our client where in they can place order.

Prior to this I built a database for our product which include price, name & etc..

Now, I want to create a form for our client that every time they select one product from "multi select" field, two additional fields will come out after selecting a product:
  1. Quantity
  2. Add more product
"Add more product" is is also "multi select" field which they can choose among our product list. Then if they select another product, "Quantity" field and "Add more product" should come out again.

Please help, thanks.