Going to go pedantic on you, but I believe you mean 'affect' and not 'effect'. I used to always have a have time with this until my doctoral supervisor told me to remember the mnemonic RAVEN: Remember Affect Verb Effect Noun. So 'to affect' is the verb and 'an effect' is the noun.

Of course, English being what it is there's an exception or two! Effect can be a verb when used in the form 'to effect a change'. And affect is from time to time a noun when used by psychologists.

But mostly RAVEN applies.