Basic list of parts:

Case: What your PC is housed in. Size dictates other parts a bit, but generally it's one of the last parts you buy.

Motherboard: central nervous system of the computer, where most of the functions of a PC run through to receive or send data from other parts. Make sure it has optical ports, as some motherboards now a days don't have them to cut costs. Should have at least one PCi or PC interface port. Depending on your board, certain processors do not work, be advised.

Processor: Your central processing unit, or CPU. The two big companies for processors are AMD and Intel. Intel is more expensive but have some more features and are marginally better than AMD. AMD are cheaper, and have a few less features, but are nearly equal in power to Intel processors. AMD are better for cheap builds or when you need to find a couple extra dollars for another part (Normally the power supply or graphics card)

RAM: RAM (Random access memory) Is necessary for computer processing. The more RAM a computer has, the faster it can process, multitask, so on, but don't spend too much on it. Some RAM is overly expensive.

Power supply: The heart of the PC. Generally, you want to go 200 watts above your estimated wattage on your power supply in case of extra energy draw or future upgrading. This is arguably the most important part of the PC, as without a good quality PSU of the right energy output, your PC will fry.

Hard Drive/Solid State Drive: the brain of a PC. Generally speaking, Solid State drives are expensive for larger sizes, but read (Send necessary info) quickly as well as write (add received data to storage) to it's memory nearly as quick. Hard Drives are cheaper, but slower, generally used for large amounts of storage these days.

CPU cooler: Depending on your CPU, you may need a cooler. Research online to see if one is necessary. Needed to keep the CPU from overheating and keeping performance good. Buying an independent cooler is most of the time optional, but highly recommended for performance. If you have a stock cooler and are fine with that, however, it should be alright.

Optical Drive: Your run-of-the-mill CD/DVD tray for the PC. Not necessary if you're capable of using a flash drive to install your OS of choice, in specific, Windows 10 is very easy to install via flash drive, if you want to use Windows 10. Other OS's may have differing methods of burning to USB. Research and see what is right for you.

OS: OS, or Operating System, is more or less your interface and computer's consciousness. Absolutely necessary, however there are tons of different OS's out there. Research and see what is right for you.

Network adapters/Cards: Generally, these parts exist for purposes that don't always exist on motherboards or other parts, things like WI-FI adapters and such. If you can, use Ethernet if you are seeking to use this as a gaming PC as WI-FI is not very advisable. Some motherboards do have WI-FI built in, so if you are seeking to use WI-FI with a build, it is not entirely necessary. If your board of choice lacks Ethernet for some reason, there are also Ethernet cards.

Graphics card: Optional part most of the time. Gaming PC's, among other things, use these to process graphical changes much more quickly than possible by normal computers without them can as their entire purpose is to process graphics. While the PSU is where you buy the most quality parts, GPX card is where you normally see the largest percent of cash flow into.

Sound Card: For optimization of sound, optional part like a graphics card for sound.

Peripherals: Keyboard, mouse, monitor, speakers, the like. Choose what you wanna use. Very opinionated, especially keyboards. If you want my opinion, Gigabyte makes some of the best keyboards around. Logitech makes very good headsets with mics in them. I enjoy shark force mouse's. Acer have some amazing monitor's. Speakers are up to choice and budget, I don't have much of a preference.

Other: Some items you will need to obtain, like case fans if your case lacks them or the ones in your case are not what you want in your build. Pay attention to your parts and what they need. If you decide to go for a non-stock cooler, in example, you need what's called thermal paste to make sure the cooling from your cooler actually affects the processor. Do research on what your parts need, or what you want your CPU to achieve.