Creating a sandbox is done by creating a new Mendix-project and then publishing it. Get access using the administrator userid and password set security in tab demo-users. Sandbox is for playing around, non-production, small environment.

Build your own environment is done by either moving your sandbox to a license node, basic, standard or premium, see https://www.mendix.com/pricing/, Usually in the Mendix-cloud but optionally on premise.

Good luck tomorrow.