Create a "transaction ID"

I am developing an online POS for my store. I have most of it figured out on paper already.  The part I am stuck on is providing a "Transaction ID" to each transaction that is processed. I have it set up in my sql database as an INT. But beyond that, I am not able to get it to increment for each transaction. It keeps incrementing for each item that is added to the customer's shopping cart.
 In short, I am looking to have a unique ID for each transaction. I appreciate any help I can get. Thanks.
...