What causes a slow idle speed on my 2001 ford focus?

I know there are endless reasons why but can someone name off a few major reason?

Nothing does… Time is slowing down in the world around you.

Your mechanic will figger it quicky. What does your shop manual say?
if you are foolish enough to work on your car with NO knowledge the book is a must. And yes countess causes, vacuum leak? You could give more info. If you really wanted a answer

Vacuum Leak, Dirty Throttle Body or IAC, Dirty MAF, Dirty Injectors, Worn Spark Plugs. Really would be good if you got a Repair Manual to help you understand your car.

A dirty totally dirty air filter