It's something you'll hear a lot if you want to be a writer: Read. A lot. This is something I've resisted somewhat because if I'm being honest, reading isn't my favorite past time. I much prefer eating cookies. But I understand the value of the idea, and I'd like to share some thoughts on the subject.
First of all, if you passed the 8th grade and still struggle with grammar and punctuation, you can learn what you need to by reading. (Excluding Junie B. Jones books.) As you read, pay attention to punctuation usage, sentence structure, and grammar. Okay, I have to make a confession here. Certain grammar rules still elude me. If this is the case for you too, instead of reading books, read grammar blogs and websites. (Here is one of my favorites.) Or, better yet, purchase an 8th grade grammar workbook. Don't forget to open it and do the work.
Second, read to study. It's great to pick up a good book and get lost in the story and characters, but if you're a writer, you've got stuff to learn. Pay attention to things you like and don't like. What works? What doesn't? What does the particular author do well? Do they add extra little details that contribute to their descriptions? Do they weave a complicated plot? How do they introduce their characters? What point of view and tense are they using? Does it work? Why or why not? What makes the book good or bad in your opinion? Pay attention. Study. Learn. Read. Read some more. There is an infinite amount of knowledge to be gained--as far as writing is concerned--by reading. So open a book, and get crackin'.
Hey, what's that cool book on the top?