I'm no expert.. I just built my rig this week
but I will share my input for my research.
Go for ATX... Look for a case that is mid sized with plenty of fans and cable management. Then you can rank them further by if want cool LED lights and form factors. Most mid sized cases generally come with plenty of drive slots and removable back plates (this is how external motherboard ports are accessed in the back). If the case has front USB and audio ports that is a plus as well.
I think the best order to picking out your parts is:
1) Motherboard - honestly, you want to save up to get a really good quality motherboard. This guy facilitates everything so don't skimp on it. Can be a pain to upgrade... good quality ones are available for under $150. Asus is pretty awesome...
2) CPU - another component that you want to get the best possible when starting out. Today you can run any game on a higher end i5...CPU can be overclocked to improve performance. A bit easier to upgrade then the motherboard, but if you get the motherboard and CPU that you want at the start then the rest of the parts can be upgraded easily later down the road.
3) Everything else. You can fill the rest out with budget components. I would recommend going ahead and getting a 8 GB ram stick instead of a two 4 GB kit so you're not using up your slots and wasting money later. Also would say get a solid power supply from a reputable company (look at reviews). Those seem to be common things that go bad when people get cheap ones from unknowns.
Lastly, I used
this site for a lot of my research, as all of their recommended components had such good reviews and I didn't want to take too many risks with my first build with no help