"According to some opinions, humanity's greatest fear is public speaking. The next greatest is dying. Does that sound accurate? That means to the normal everyday person, if you show up at a funeral, you're better off in the casket than doing the eulogy." John Steinbeck, Novelist
When you are writing or speaking in public, effective communication starts and ends with the what -- the content. The how will come much easier. Once writing training and presentation skills training help you in gaining power (and you will have confidence because you'll be a trained pro) you'll have no worries writing or speaking in public.
To get there, aided by writing skills training, remember the three pillars of effective business communication, whether written or spoken:
Writing is thinking. Compared to an unplanned conversation, writing -- whether a presentation to a group or a report for the shareholders or a sales proposal -- gives you the time to show how smart you are.
Know your audience and your readers. Are they advanced or beginners? Are they experts? Do they want to be amused or informed or persuaded? What do you want them to take away from your report?
Edit and rewrite and, when public speaking, practice. If you do not check your work with care before hitting the "send" button, or rehearse the style and delivery of your presentation, you risk looking unprepared and unprofessional. It's called quality control, and without it excellent communication is nearly impossible.