Okay, so lets cover choosing your CPU (Central Processing Unit), or "Processor". This is essentially your Machines Brain. Without this, you Machine is pretty much just an expensive paper weight. As mentioned in an earlier Post (All PCs are not created equal), not all CPUs will not fit all motherboards, So you'll have to do some research. Your first place to look, is at the specs for the MOBO that you have selected. These will tell you what "Sockets" your MOBO will accept.
If you are upgrading your CPU, then you must first refer to your MOBO handbook. If you do not know your current MOBO information, then google search for a pragram called "SiSoftware Sandra Lite". This is a free software, and will tell you each and every detail about your MOBO and PC that you would ever want to know.
When choosing a CPU, you have to balance budget, quality, and performance. Personally, I try not to buy any PC or MOBO parts used, or refurbished. And try to get something with a good warranty. Sometimes things go wrong, and you want to be covered. Generally the higher the GHZ (gigahurtz), the faster and stronger. However, with CPUs, some are better than others. So be sure to look around.