There are different ways of running an art shop. First, you need to decide whether you can take on the responsibility of an art shop, and whether your art is good enough people will pay pets for it. You should be able to draw lines well and smooth, shade properly and use Lock-Alpha. Then, decide what program you want to do it on. It should be a program you normally use and know best. There are different ones, like Adobe Photoshop, SAI, SumoPaint, GIMP etc.
Regarding your questions:
1. You create them by drawing them on an art programme, then post them in your art shop.
2. You recolour them by colouring them in and using different colours/patterns etc. If you use a different paint tool to ChibiPaint(Oekaki), you have to save the lines and then create new versions using them. You should use layers and lock-Alpha, because it will make everything much easier.
3. You post them and let people adopt them by filling out a form you think is appropriate for your art shop and (maybe) paying you pets, depends if they're free or not.
4. You can post in the title of the thread something like : "Hiring artists!", "Looking for staff", "Need artists and mods" etc. You own a store by making a thread, selling/adopting out pets and making new ones often for people to adopt.
Hope I helped.