Tabs indicate that there are different views of related information that can be swapped between at any time. Showing tabs and not allowing them to be used adds cognitive friction.
I wouldn't show the tabs at all until after a selection is made on the first screen. Be sure to keep the first screen as simple as possible and only ask for the minimum amount of user input (in your case initial selection). Additional information can be shown on the first tab along with the other tabs now showing across the top giving added confirmation that the first selection opened up additional possibilities.
If getting rid of the tab control until it makes sense isn't possible then make sure to be explicitly clear to the user when the action they are trying to perform isn't doing what they expect and how to remedy the situation.
A Notification has the advantage of not getting overlooked but just make sure it is direct and to the point...
"Select a product from the current tab before switching." [ OK ]