One of the biggest problems that I had when I set up my first site was essentially that I had no idea what I was doing. This manifested itself most obviously when it came to installing scripts for my site. I'm not a programmer (far from it) and I thought there would be assumed knowledge when it came to getting things running - knowledge that I assumed I didn't have.
SSI stands for Server Side Includes. In it's simple form, it means that you can put a tag in your HTML pages that will be replaced by the file that you specify. This is extremely useful and I strongly urge you, whether you are a beginner or advanced designer, to use it. It will save you many hours work later.