Choosing between affect and effect can be scary. Think of Edgar Allan Poe and his RAVEN: Remember Affect Verb Effect Noun. You can't affect the creepy poem by reading it, but you can enjoy the effect of a talking bird.
In everyday speech, affect is a verb. It means to influence something, such as in the headline from the Albuquerque News:
Downed Power Line Affects PNM Customers
The downed power line had an impact on some power customers: they were without electricity overnight.
Effect is most commonly used as a noun meaning "the result or impact of something, an outcome." If there's "a/an/the" in front of it, it's an effect. Here's an example:
"Did you have a significant effect on the development of some strain of philosophy that profoundly impacted the world?" (Made You Up)
Adding to the confusion, effect can also be used as a verb to mean "to produce or to cause to come into being." Here's an example that uses it correctly:
A government unable to effect any change is a government that will produce no surprises. (The Wall Street Journal)
Put another way, a government that can't produce change won't be able to produce surprises; it will be predictable.
Most of the time, you'll want affect as a verb meaning to influence something and effect for the something that was influenced. The difference between affect and effect is so slippery that people have started using "impact" as a verb instead. Don't be one of them! Another trick is to remember that affect comes first alphabetically, and an action (to affect) has to occur before you can have a result (an effect).