Despite my efforts, Im failing!

Im trying to set up a system whereby I can have all the products on the website, and their pricing, so that I can have an auto-populating invoice.
So for instance...
Column 1 = Quantity
Column 2 = Supplier
Column 3 = Product
Column 4 = Price

Enter in the Quantity of the product Ive sold, then type the first letter of the supplier and it auto populates (or a drop down menu), then as a result of the supplier the products from that supplier become the choice for the Product Column and once the product is selected, the price appears in Column 4....and obviously then it multiplies Column 1 and 4, and at the bottom it does the Total and VAT, etc, with the option for Discounts etc....the last bit isnt a problem! But the other bit is!

Any Excel Whizz Kids on the site?
Would appreciate the help, as it would save quite some time to have a 3 or 4 click system, opposed to having to type it all out!
Cheers!