The reason that the GUI is not appearing is that recent CentOS installations default to "minimal", installing only about 200 packages for a basic system, and it does not include the GUI desktop.
Regarding your first question, I've seen posted many methods to achive your goal. Here's one possibility:
yum groupinstall "X Window System"
yum groupinstall Desktop
Regarding your second question. Why do you need to install yum? It is part of the default installation no matter if that installation was minimal or full or anywhere in between, so it should already be installed. Is yum not working?
Regarding your third question. Installing hardware drivers is specific to the type of hardware. I have no idea what hardware you have.