You can rent a plot in the mall to open a shop. To get there, use /warp mall. Once there, you can look for a plot that you like in one of the four districts in the mall (Red, Blue, Yellow and Purple). If you found an unclaimed plot that you like, walk up to the floormap of the district (located between the /warp spawnpoint and the color district your desired plot is in), and right-click the sign that corresponds to your plot. Small plots cost 1000 DCD per month, the double-sized plots cost 2000 DCD per month. You can rent up to at least 6 months, if I recall correctly. You can extend the rent at a later point as well, in the same way. If you don't extend the rent of your plot, it will get fully reset when the rent expires.
To make a shop, place a chest (regular or trapped) and leftclick it holding an item you want to sell. It will then ask you to enter the amount of DCD you want to sell said item for. Enter that, and a chest shop is created. You have a limited amount of free chest shops to begin with (from the top of my head, 10, but I could be mistaken), and you can buy more in the shop on the website (on the left, Shop > View Stock).
Looking forward to seeing your shop!