What you have been writing online so far are personal remarks or opinions. For professional writing, when you're objective in reporting facts and recounting situations it'll be easier to keep cool. There's also the editor to backstop you. If there is anything a professional writer should know though, is to ensure the difference between personal observations/opinions and facts. Good vocab and proper sentence structure needless to say. Mainstream media is more about recounting facts. Only senior writers or editors get to be creative, so you needn't worry.