Format and style are very important when writing your dialogue. Use quotation marks ("...") to show your readers that the words being read are spoken directly by the characters in your story. For example, the following sentence lets us know that someone is speaking to Johan.
"You are my best friend, Johan."
Always remember that punctuation goes inside the quotations.
For more information, read Fokus SPM, April 2013 (page 44-45).