The correct word to use in "The fireworks exhibition had awesome _______" is "effects."
In everyday speech, affect is a verb. It means to influence something.
Effect is mostly 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. Adding to the confusion, effect can also be used as a verb to mean to produce or to cause to come into being.
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.
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). Check out some grammar rules and grammar tips at Grammar.yourdictionary.com and get 8 tips to help you master affect and effect at Poynter.org.