Create a same signup flow concept as EDUBlogs

We have setup 4 demo packages. When you select one package, currently you are being redirected to a login page.What I would like to accomplish is the same effect as edublogs. (http://edublogs.org/signup/?Pro=1)That the user can directly create a site and then continue to payment.Can you point me to the right direction on how I should set this up. Maybe also a good question, which plugins are essential for this flow?