What's your CPU? AMD or Intel? What model/pinout/socket-support? Overclock or not?
What's your case? ATX? mATX? ITX? Does it need USB3.0 or only USB2.0 front panel connections?
What's your GPU/s? Single GPU? Multi-GPU (SLI or CrossFire)?
What's your RAM? Single? Dual? Quad? What speed?
What's your PSU? How many ATX12V/EPS connectors does it have?
How many case fans will you install? Will you need AIO/liquid cooling and pump support?
What other connections do you require your motherboard do have?