The blank page. Starting is usually a process of thinking through the details, and using speech-to-text apps so that I can get words on that first page and then edit those words. Editing always comes easier to me than writing.
I think I started writing because I'd have these images or stories in my mind that I wanted to preserve. My memory is terrible, so I'd start writing them down. I wrote cheesy little murder mysteries when I was a kid. But I got away from writing (and reading) for some time and spent my time on photography instead using that as my preservation method. Then, in 2003, I went to Budapest and fell in love with that city. There are aspects of the city that the camera just couldn't capture, so I started writing about it. I didn't have a story to tell, just experiences and scents and feelings to remember. It wasn't until I went through my own open-marriage and divorce with my first husband that I was inspired to write the Lost in Austin series. Since then, stories clog my mind waiting for their turn to be written down.
Use every tool at your disposal. For me, I found that my mind would blank when I'd sit down at my laptop, so I started using a speech-to-text app on my phone so I could dictate my story, or a bit of dialog, or an outline while I walked around. Then when the words were on the page I could play with them, move them around, improve and build on the story.
So my tip is: If you're struggling with any aspect of writing, consider what tools might help you with that part.
