Well first you have to determine whether you want to build your own or get a prebuilt system. Here are the pros and cons for both.
Pros for homebuilt
-Individual parts are often cheaper due to deals and rebates.
-Higher quality overall
-Personalization
-Sense of accomplishment when you hear that post beep and the bios appears
cons
-can be more expensive as a whole
-difficulty in the actual build
-many different companies to deal with if there's an issue
-there's way too many parts to choose from, and it can seem a daunting task
Prebuild system pros
-often cheaper as a whole
-you don't have to build it
-you only have to worry about dealing with the company that assembled the pc, not the companies that made the parts
-simplicity
cons
-they often skimp on the power supply and don't use a good brand
-you don't get to decide what you want
-you don't have a choice on the case (the most builder-specific component)
-less options
-doesn't make use of individual component's sales
-usually comes with a really low end gpu
Overall, I feel that building your own pc both gives you the system you want, and one that you can take ownership of, because you actually built it. If you find a good prebuilt pc at a nice price, that's great, and you should probably get it, but I like knowing what's in my case and that I put it there, not some anonymous person I don't know.