Perhaps you want to get into the habit of writing regularly (maybe daily, maybe a couple of times a week) ... but you're not sure what to write about.
It's worth thinking about why a regular writing habit is important to you. For instance, maybe:
#1: You want to improve your writing (e.g. to help you get a particular job or take a class)
#2: You want to write novels or short stories, either just for fun or for money too
#3: You want to build a blog to support your business
#4: You want to write for personal fulfillment – perhaps a memoir or journal to pass on, one day, to your descendants
#5: You want to write because you find it therapeutic and it helps you work through problems
Your reason for wanting to write daily makes a big difference in how you go about it – and in what topics you choose.
If you want to write a novel but you haven't settled on a genre, or you don't yet have an idea for a plot or a main character, you could:
- Read a lot, and ask people for their recommendations: I'm not suggesting you write something really derivative, but what you're reading could spark an idea.
- Think about the sorts of characters you're interested in writing about: some novelists start with characters and build a story around them.
- Stay alert for anything that pops into your head and makes you think "Ooh, that could be an interesting story..."
While it's pretty easy to just come up with ideas, coming up with an idea that you want to write about is a fair bit harder! It may take some time for the right idea to germinate, and if your ultimate writing goal is a big one (e.g. a whole blog, a novel, a non-fiction book), you might want to work on some shorter projects in the meantime.
One important thing to say here: there's nothing magical about writing daily. For some writers, it's a good and helpful habit: others do perfectly well writing a couple of times a week, for longer sessions. Experiment, and stick with whatever works for you.