You can do what I do. Buy an expensive prebuild PC, mine is 1100 Euro. Then when it gets old, just replace the part that is necessary. For example, I was playing Skyrim, and I was tired of the low FPS, so I bought the cheapest GPU I could find, but one that is still better than what I had. That way I have a good computer, and by replacing part I learn slowly how to assemble a PC.
Now I have problems with my CPU, I'm doing too much ate the same time. Heavy gaming and 100 tabs open at the same time. So I will be learning how to install a new CPU when I got time. And just like that, I learn slowly and easy how to assemble a computer.
(My new GPU was 150 Euro btw, and I can run Skyrim at max with minimum hiccups. But I never play at max.)