If it's a makeshift, cheaply-made spacecraft, it will probably look like exposed isogrid. Isogrid is a thin skin with stiffening ribs, often used in spacecraft (especially pressurize structures). Here's a picture of some isogrid:
http://isogrid-sst.com/Local_View_Isogrid.JPG
If it's more luxurious, there will probably be padding on the walls to prevent things from banging into them.
If the spacecraft is manned, expect instrument panels, a computer terminal, and a life support system. There will be a steady hiss and hum of air and coolant moving through valves.
