A touch of dialect helps establish voice, and may lend authenticity to the writing. But for the beginning writer, knowing how much is enough is often difficult.
A story studded with apostrophes and phonetically spelled words draws attention to the writing, detracting from the story. Two of the most frequent verbal habits are saying an’ for and and dropping the final g from words ending in -ing. These words occur so frequently that the printed page sprinkled with apostrophes looks odd. Not putting in the apostrophes for dropped letters–for example, simply writing an instead of an’ for and–may actually be confusing.
A better approach is to look for a few places where phonetically spelling dialect makes a difference and drop out all of the others. For example, the difference between boo-kay and bouquet is so subtle that it probably isn’t worth making the reader pause and notice. Instead, rely on vocabulary and grammar to establish voice.