The first decision when assembling your own PC is what form factor to use. If you're starting from scratch, the best choice is to follow the ATX standard for cases, power supplies, voltages, mainboards, etc. Mainboards for these old styles are still widely available, although perhaps not for much longer. With your form factor selected, now shop for a case. For servers, don't get anything less than a full-size tower.
Some people prefer a minimalist approach.
They get the smallest case that will meet their basic requirements. Now the power supply. A 250 watt supply is more than adequate for most desktop boxes. For servers you'll want something a little beefier, like 300 watts or more, to drive those extra cooling fans and deliver the amperes to all those hard drives when they're first spinning up. Next you need to make a decision about the CPU, because this effects your choice of a mainboard.