1. I suggest finding a processor A.K.A. CPU first. It is one of the most important parts because it is the central processing unit. Intel is good if you have the money but with AMD you usually get what you pay for. Although a AMD 8 Core cost about the same as a Intel 4, the Intel usually performs better and last longer. Majority, if not all, Intel CPUs come with built in graphics. Not many of the higher end AMDs do though.
2. The second thing to do is find a motherboard that matches the CPU that you picked out. The way to find out is on AMD CPUs, the socket may be AM3+ but with Intel it may be 1150. If you going to OC (Overclock) make sure that it is meant for gaming and also check the reviews. Make sure it has multiple video ports in case you want to expand. It will most likely be PCI Express 3.0 x16 or PCI Express 2.0 x16. The motherboard will usually cost less than the CPU unless you want the best motherboard.
3. The next thing to pick out would be the graphics processing unit or GPU. It is also considered a graphics card. If you have a graphics card built into the CPU, try and find one that is compatible with the graphics card to use it the most efficient. Games like WoW dont need alot of graphics to run but the newer games like Battlefield and Call of Duty require alot faster and better cards. Also while finding GPUs, find out how new they are. Older cards may not be compatible with newer games. Usually the new Radeon Cards will be the R9 or R7 series. The newer/ better cards for Nividia will be the GTX 600s or GTX 700s.
4. The next part to find will be the Memory or RAM. There is a huge difference between Memory and Storage. Memory is what it processes almost. It will speed up basic stuff. The average for gaming is 8GB and up. You will have to check how much RAM your motherboard supports, usually 32GB. Also, you have to check how fast which most AMD motherboards are 2400 DDR3 and some Intel support all the way up to 3000 DDR3. It does not matter alot how many slots it takes up except that you may want to later expand.
5. The next part, and almost final, is the Storage which is usually in SSD (Solid State Drive) which is more reliable and the HHD (Hard Drive Disk). The SSD is more reliable but is alot more expensive. It is smart to buy one to put your OS on so that it runs slightly faster. It is ideal to have about 1TB worth of storage. You also need to make sure it uses the same connector which is usually SATA.
6. This is one of the most important parts, it keeps the computer running which is the PSU or Power Supply Unit. To find out how large of one you need, there are multiple websites that help you figure it out. Usually the minimum will be 500w. It also depends on if you are going to Crossfire or SLI which is when 2 or more graphics cards working together. Make sure the plugs for the PSU match your motherboard and other components. Read the reviews because some PSUs dont last long or explode.
7. One of the unimportant parts, the DVD or Optical Drive. Although many computers have it, there are other ways to use a computer then to have it. This is also probably the cheapest part you will need.
8. The final part, the case. Although this does not seem important, it holds everything together, you need to make sure it fits your Motherboard, GPU and PSU. Also the case needs to have enough fans to create enough airflow to cool the components. As learned form experience, you need to add fans unless it has more then 1 already in it. If you do not, your computer will overheat easily from long time gaming sprees.
Thanks for reading this Tutorial, I hope this helps many figure out the basics of building and designing computers.