Shops are fairly easy to create. The first step is to craft and place a chest, like so:
Next, crouch (shift) and left click the chest. A message asking you how much you want to sell one item for will appear:
Type a number into chat for the price you want to sell things for. Then a sign will appear, allowing people to interact with your shop.
People can now right click to start a transaction. If you want to change the shop's settings, right click the sign. A panel will pop up with different options allowing you to change it to a buying shop, refill the shop, change the price, or extract items from the shop.