A purchase order tells a vendor exactly what you want to buy, at what price and by when. Here you are the buyer, so your business goes at the top and the vendor below.
A purchase order goes from the buyer to the seller before the goods arrive. An invoice comes back from the seller asking for payment. The vendor usually quotes your PO number on their invoice, so the two can be matched.
Once the vendor accepts a purchase order, it is normally treated as an agreement to buy on those terms, so check quantities and prices before you send it.