First you need items in your inventory that you want in your shop! Then you have two options:
You can click on an item in your inventory, and the menu will say "Move to a Shop" at the very top. Click on that and it will give you a list of your shops. Then click the shop you want to add the item to. It will be in your shop now.
You can go to Your Shops Page under the commerce tab at the top of the page. Select the shop you want to stock. The menu on this page underneath "Your Shop: Shop Name" will have a link to a quick stock page. From there you can stock multiple items at once from your inventory.
Once you've got items in there make sure you set the prices, then your shop is stocked!
It looks like all of your current shops are on the old system. A couple years ago they updated the shops and any old shops can no longer be used. You will probably have to delete one of your shops and make a new one to add items in again.
It looks like you still have very old shops though, and they were changed to a new system. So first, you'll have to follow this guide to help you make new shops: New Shop Guides