First you will place the chest down where you would like the shop to be located. From here go ahead and place your sign on the chest like in the picture below.
On the first line of the sign leave it blank. On the second line place the quantity of the item you are selling. On the third line either put “B” for selling your item to a player, or put an “S” for buying items from a player. You can also put both if you would like. On the fourth line put the item name. You can find the official name of an item using the /item info command.