This is where I get my parts. You'll need a CPU, GPU, SSD, and Ram. There are other important parts but these make a difference between a PC and a gaming PC.
The CPU is the brain of the computer essentially. They think and send directions to every part of the computer. The faster the processing, the better for gaming. The better the CPU though, the better of a heat sink and cooling you need to make sure it doesn't overheat.
The GPU is like the CPU but for graphics. It accelerates generating graphics and takes a load off the CPU boosting FPS.
The SSD is a hard drive that doesn't have moving parts and is usually faster at reading and writing data than a HDD. Now RAM, it temporarily holds cache or data that programs may/will need soon. As for the SSD and RAM. They're pretty much memory.
There are more important parts that affect how fast and reliable your PC is like the cables used, heat sink/cooling and fans, as well as a motherboard. Talk to a pro if you want to build one yourself and want to get into details because thats just the gist of it.