First of all, I would recommend buying a shop when you first get on, because they are bought fairly quickly, but make sure you have enough money. Here are the steps.
1. Buy the plot. Search for an open plot on the mall. Go up to the sign and click on it. It will cost 1,000 Dcd.
2. Decide what you are going to sell, (Dirt, Sand, Wool, etc)
3. Place down a chest
4. Put a sign on it
5. Click the chest with the item you want to sell
6. Decorate your plot in order for people to notice and want to go in
7. Bathe in your profits
Heads up, when you first make one, barely anyone will buy anything. Make it bigger, flashier, and sell more stuff.