Case
The case is going to be housing all of your hardware, so its important that when you come to put it all together that it all fits. You can measure your parts up to the dimensions of the case listed on the website to make sure all of your parts are going to be compatable. Its also important to think about how you are going to cool your system, if youre aiming for water cooling you are going to require a specific case with holes to run the piping through. Where as if you are air cooling its important to look at how many fans you can fit into the case and how hot its going to run. Something you may also want to consider is the USB ports, many but not all come with USB 3.0 on the front of the case and its a very important feature that should be included.
When connecting your PC to the internet, the two most popular choices are an ethernet cable which will provide the user with faster speeds and a more reliable connection. The downside to this is you are going to have a cable running from your PC to your modem/ router. This is why some people choose to use a WIFI adapter for their PC, it requires no cables and connects directly to your motherboard, and then to your wifi to provide your pc with an internet connection. It may not be as fast or as reliable as wired but its a hell of a lot easier.
Connectivity