First thing you need to do is to craft Trade Shop block. You can do it with any 4 planks, 1 wool and 1 iron ingot. 

Then you need to place it against some container (either on top of one or on the side).
After that, Shop Settings will open. You need to set up things like what type of stock is sold and the price. Optionally you can enable support for hoppers or disable hologram. 

To change settings, just click item corresponding to them. In case of item based one you can set them by clicking (empty/set) slot. If your cursor holds item, it will be copied as a settings. Otherwise, it will open menu to set it manually. And done, shop is fully operational. You only need to put required stock in your container and others can just buy it. 

To buy things you just click to open the shop and then take sold item. You can hold shift to move it directly to your inventory!