Dialect and accent are wonderful spices for dialogue, but just as in cooking, you don't want to add too much. An entire dialogue spelled phonetically is just too hard to read. Do your research and find out some common slang, idioms, and phrases that a native speaker would be likely to use, and sprinkle those throughout the (properly spelled) conversation. Add a dash of phonetic spelling once or twice in the story if you wish type my essay, but no more than a couple of words just to drive home your point. Remember that the point is to drive the story forward, not bog it down by forcing the reader to translate dialogue into understandable English.
Make your characters unique
Everyone has their own way of speaking, and your characters must be individuals. Think about each one. Is he an educated man, with a habit of using long words and lengthy sentences? Does she speak slowly or quickly? Are his sentences short and bitten-off? Are her conversations peppered with curses, or does she use euphemisms? Your readers should be able to identify the character by the way the line of dialogue is written, without needing more than the occasional "Bob said" or "Sally replied" added.
6. Avoid the "info dump."
Many novice writers try to cram too much information into their dialogue, with the result that the characters end up droning on and on like boring professors. You've all seen old B-movies where the scientist says something like "As you know, Dr. Smith ..." and proceeds to spend five minutes lecturing the other character. Not only is that boring, it's illogical. If Dr. Smith already knows it, Dr. Jones wouldn't need to tell him. Don't let your characters say things that they wouldn't really say, especially if it turns into an "info dump."
Once you finish your story or novel, you'll edit in several "passes" (read through the manuscript looking for one particular thing at a time). One pass must be for dialogue. Cut anything that doesn't move the story along, no matter how great you think that line may be. If it's just talking for the sake of having dialogue, it doesn't belong in the story.
All you need is a bit of practice and some concentration, and your dialogue will sparkle!