First things first. If you're serious about this buy the book Earthbag Building by Kaki Hunter and Donald Kiffmeyer.
Buying and reading the book will be much simpler than me trying to explain everything here.
How you will want to design your shop is up to you, but if you want to use on site soil, you will have to bury it a bit. But in consideration of your water table, you may be better off having road grade trucked in and building up.
For the size of building you wish to build, just remember that it will be very labour intensive and you may spend the better part of a summer or season building it unless you have very, and I mean VERY good friends. Or you could throw a workshop

In my experience, workshops are great for getting free labour, but you should charge people to attend or they won't show up. When people put a monetary value on the things they learn they are more willing to do them.