Thanks for reaching out to me, Annie! I apologize that it took so long for me to get back to you.
I wholeheartedly encourage you to start writing here on Medium. The great thing about it is that you don't have to be an established writer. You can write whatever you want, and it's out there for others to read.
My advice about writing is simple.
No matter what you'e writing, write like you would speak. I hear every sentence I write in my head, and make sure it sounds like something I'd say. This way, I'm able to establish a style and voice more easily.
Write about anything that interests you. If you have a personal story you really want to flesh out--do it. If you have an opinion you'd like to get out there: do it! Don't limit yourself--especially not at the start.
No one is a good writer when they first start out. So get that idea out of your head. Just try to write regularly. Also, get in the habit of thinking about writing when you're doing other mindless things.
Also, capture every idea you may want to write about it in one place for later use. This is responsible for some of my best pieces. I was doing something, I came up with an idea, and I recorded it in a list of ideas. Later, while reviewing, I saw it, picked it out, and made it a piece.
That should be enough to get you started.