first step - we have to consider the funtional part of the case,letting stetic aspects to the end of the matter. Based on hardware you are planning to use you most select from MINI,MID,or full tower, as most used sizes, and this aspects:
-Interior Space( to work confortably)
-Cooling system(More fans the better)
-PSU mounting (Bottom mounted is better for cooling).
-Cable managment(also better for cooling).
-Supports for long graphics cards(for not getting incompatibilities issues on future).
-Water cooling possibilities (in case you practice extreme OC on the future).
-Tool-Free mechanical design (for quick assembly and mantenance).
-Dust filters on fan mounts(to prevent massive entry of dust into the case)
-there are others things to take care, but with above you will be safe
Second step-Stetic aspects(acording to each one point of view)
-Interior painting(black coating)if possible
-transparent side window
- the global view of the case
-and other Mooding things