I just got this diagram in my attempt to answer the above question. http://doityourself.com/images/housenet/waters.gif
There is a screw on the top of the fill valve, but these are usually corroded. Most people just bend the rod holding the float to adjust the water level.
Check the flapper and re-adjust the fill height.
Yes, adjust the fill valve, different ways for different types.
If it's a normal household toilet you simply lower the water level in the tank that's over and behind the toilet by adjusting the fill valve. There are many different designs so I can't tell you how on your particular toilet. Some ae as easy as turning a plastic screw and some are a real pita. If you adjust it and raise the water level too much water may run over into the overflow tube and it will just keep running. If you lower it too much your toilet may not flush completely. Generally I adust mine as high as they will go without overflowing in order to get a stronger flush. I have never needed or wanted to turn down a flush.
Which part, the tank or the bowl? If it's the tank, you have a problem with a restriction between the tank and the bowl. If it's the bowl, you have an obstruction in either the neck of the toilet or the drain pipe below the toilet.
What exactly is overflowing? this can range from a blocked toilet to a faulty washer
To allow toilets to fill from the water line without overflowing.
its a squirrel
no
There should be a screw at the valve end that can be screwed in and out, but it is usually so corroded that you can't. Most of the time, you just bend the rod the ball is on to adjust the water level.
Yes, so you fill it up to the fill hole then screw the plug back in.
The main toilet pipe is partly plugged after where the upper pipe joins the lower one.