What You Should Know
The good thing about building your own PC is that you get to enjoy the step-by-step process of customizing. Useful information such as how to purchase the parts, as well as what to do on the 'big day' of actual assembly of your PC.
Purchasing
You can spend as much or as little as you want at a specific price-point and can have vastly different computers that can be good for one use and lacking at another. Computers are tools. You need to be sure to get the right configuration for you specific needs.
Preparation
You will need roughly a day to build, install, and configure your new computer. Also, be in a quiet place, a playroom with a 2 year old is not a good place to build a computer.
Open boxes and put everything on a clean table. Do not take the components out of the anti-static bag. Yet Be careful not to mix the screws as they might look the same, but might not have the same threading.
Keep the manual. While this tutorial will teach you a lot, it simply cannot give you all off the possible information about all of the potential hardware that may be used.
Building Day
Try to set aside a day to build and install in one day, leaving a computer unfinished and coming back to it the next day is a good way to forget something. Do not begin to build a PC before you have all the pieces on hand.