You might start by deciding if your a fan of Intel or AMD and how fast a processor you want.
Then decide on a motherboard supplier and you will have MB choices based on your choices above.
Within that MB range you can pick how much support you want for growth (Sata, raid, GB lan etc).
Pick memory based on what your MB will support.
Pick a quality PS that will handle your plans for growth.
Basically you makes things compatible in the process of making your choices.
If this doesn't sound like fun let someone else make the choices and explain them to you and build it.
Last choice a commercial vendor. Power supply ? MB ? Memory ? Proprietary parts? Price of parts is a major concern to these vendors. Since quality and price generally rise together you will only get what they think you need so they can protect their profit margin. They want you back in a couple of years for a new machine.
Or you can be upgrading your perfect computer by utilizing the MB options you selected, swapping out a faster CPU or worse case the MB, CPU and memory (down the road).
Don't pay to much, technology will eradicate value quickly (the leading edge will cost you dearly - MB may be the exception here).
Great Hobby enjoy