To get a shop, simply ask a Head-Admin or higher (for now just ask KingKappa)
Once you have been given your shop, walk into it and do the command /shop browse to see the following:
The following commands require you to hold one of the item you wish to edit:
To add new items to the list do /shop item create.
To stock that item, hold it and do /shop item add.
To remove stock do /shop item remove, to remove the item from the shop do /shop item destroy (or click the underlined Remove on the Manager page.
To change the Buy or Sell price for your store, click Manager.
To change the Buy or Sell price, just click the underlined part of text.
To clarify; the first number is the stock number, after that is how much ONE unit of the item will be sold for, the second item is optional- how much YOUR store will BUY the item for.
From here, you can add managers (who can only add or remove STOCK), deposit money into the shop to allow people to sell items to you. Options are available to change the store name, Owner and also destroy listings.