Show different speech patterns and word choices in ways that are easier on the reader. Manuscript format is the most trivial and easily learned aspect of writing. Spelling and grammar are the next most easily learned aspects of writing. As long as you're learning the easy stuff, look up a word the first time you use it whenever you have the slightest doubt about it. Here are some of the errors I see most frequently in manuscripts from beginning writers.
Some expressions live on through inertia rather than accuracy.
Look carefully when you're reading published fiction by a pro to see how dialogue is punctuated. Set off direct address with commas. Semicolons are not interchangeable with commas. Samantha could tell it was going to be a beautiful day. Don't slice complete sentences together with commas.