timing

I was reading through www.sentra.net. And it says something about adjusting the timing. What do I need to do that? Anything special? I was about to go searching to find the distributor because I haven't done alot of looking under my hood. How do I know that I have a GA16 in my car. There was another type in the manual that it could have been. Is there a good diagram of the GA16 anywhere that I can use as a map? Oh, also does anyone know where I can get a good 8-ball shift knob. I bought one from wal-mart but it sucks. I'm using it for now but I want to loose it and find another as soon as possible. Thanks.
Peace,