When my students tell me they don’t know what to write about, I tell them to just start writing.
That’s the trick. If you’re wondering about how to start that story, book, blog post, or tweet, all you have to do is start writing something. You’ll be amazed at what you had inside of you.
That’s not to say it will be a masterpiece. (It might not be considered by others to be the most elegant tweet ever offered on Twitter.)
Who will be amazed is you. This is because you’ll recall how previously you kinda/sorta wanted to say something but didn’t have the words. Then the idea came to you from somewhere when you started typing, and now you’re seeing it fully articulated right in front of your eyes on the page or screen.
Start writing. It might not be perfect, but it will certainly be worth it.