When is it more cost effective for someone to build their own PC versus buying a computer already built?
user
Building your own PC is the best solution for those who want full control over every aspect of their build. It provides the most thorough customization options, from the CPU to the fans and lighting. From this perspective, it is generally cheaper to build a heavily customized PC yourself rather than contracting someone else to do it, however it is likely to cost more than a basic pre-built PC.

Here are some of the top benefits of building a PC:
      1. Cheaper Long-Term. Initially, building a PC is typically more expensive than buying a pre-built machine. When purchasing components individually, however, they are often better in quality than the bulk-ordered components that go into pre-built computers. This leads to an 
          overall better build quality that makes the computer have a longer lifespan. Building a PC will actually save you money in the long run, because you will likely not need to replace or repair components as often as with a pre-built.
      2. Easier to Fix. When a component fails inside a PC you built, it is easier to identify because you are more familiar with each part. When you buy individual computer components, they often come with extra spare parts that pre-built computers do not. These additional parts 
          can sometimes save you from costly repair bills or unnecessary tech support visits.
      3. Better Overall Quality. If peak performance is your goal, build a PC. Building a PC allows you to handpick every component that goes into your machine. When you have total control over your computer's internal components, the final product can have a better overall build 
           quality. Pre-built PCs often focus on just a CPU or GPU and fill the rest of the computer with cheaper, less desirable components.

Building a PC can also be a rewarding experience that will help to foster a deeper appreciation and understanding of the deeper workings of your PC.
assistant
You say that it is actually cheaper to build it myself, but I don't know anything about PC. Rather, I don't know anything related to electrics! I would spend probably 1 week just to figure out what are the correct cables. What should I do?
user
Figuring out cables would take perhaps few hours (in total, for all components), not a week. 

Start by defining your budget, then take approximately 25% and choose best CPU for that price, and 25% & best GPU for that price. Afterwards, select a motherboard, which can host your CPU (motherboards have CPU-specific chipsets so it is the main potential failure point). When you are done with these main parts, throw in some RAM and hard drive (motherboard description will directly tell which kind it supports). Finally, add a power supply to keep things running, choose some good (though they are usually cheap) CPU fan and select some case to put all the goods in (larger cases are better since they guarantee you wouldn't have problems fitting all the stuff inside). 

Finally, ask the shop support whether the configuration you collected makes sense and whether components you have chosen would properly interface with each other (aka basic sanity check).
assistant