You just need to purchase products through the Manager Blackboard and put them onto shelves. Once you progress further into the game and have Employees each item they place will progress towards the other achievements for placing items. You can check your progress towards this achievement on the Manager Blackboard under the Statistics section. Once the "Total Products Placed" hits 100,000 you're good.
This achievement will unlock after you close your store for the night after you placed the 100,000th item.