Grammar is something we as children have forced upon us when we believe it isn't that important. As storytellers, we like to excuse bad grammar because it is part of the story, but unless it is part of your dialog, bad grammar is bad grammar. People may not speak in complete sentences, but we do read in them. Nothing takes a person out of you story than poor sentence structure. If one has to read a sentence more than once, you have lost your audience. A well written sentence is key to telling a good story.
Likewise, spelling and word choice is very important. In the experience of the demented elves, a spelling error isn't always a misspelled word. Sometimes, a misspelled word spells another word that changes the whole sentence. For example, the tone between respectfully and resentfully (a time when a spelling error led to a spell check error) changes the entire sentence. ( WATCH OUT FOR SPELL CHECK AND AUTO CORRECT! These technologies are as much a hindrance as a help.) As for word choice, know the difference between words such as then and than, effect and affect, and other commonly confused words. Wrong word choices could make you look like you don't know what you are doing and there goes the audience, on to someone else.
Then again, one must not simply use punctuation, but must use the correct punctuation. The demented elves once read a book with no punctuation. To be honest, This elf never finished the book because it was too hard to read. One could never tell if one was reading action, description, or dialog. This made it difficult to know what was going on in the story and thus it was never finished. While the need for punctuation is important, it should be noted that incorrect punctuation is as bad as no punctuation at all. The wrong punctuation can change the entire meaning of the sentence just as the wrong word can. As the internet is fond of pointing out, there is a difference between "let's eat, Mom," and "let's eat Mom."
If you have difficulty with the basics, and even if you are not, it might be in your best interest to invest in a book on grammar. Even those well versed in grammar need to reference the rules from time to time. The demented elves believe it is better to check than to be wrong. A well written story is necessary for a well told story.