You can do many things to help your child become a better writer. Here are a few ideas.
Encourage them to write as much as possible and it doesn't always have to be stories. Any type of writing that requires them to take their thoughts and put it down on paper for someone else to understand will benefit them. Below are some ideas that require students to think about what they want to say and write it down so someone else can understand it. All kids have wonderful ideas in their heads. The difficult part is teaching them how to transfer that to paper.
Write down a shopping list or things they need from the store
Write a note or message to a friend or family member
Write down something they want to tell you
Keep a journal or diary of their own
Build their confidence by praising any moments of success (small or big).
When they ask you how to spell a word...have them find that word somewhere they have seen it before or tell them to write down the sounds they hear. Just like reading, students should write their prediction of the word and then reread it to see if it looks and sounds right.
Have them keep a list of events or ideas they could write about in an easy to remember spot (ex. refrigerator, notebook, etc).