I recommend starting a list of topics in a to-do list app like Reminders. You can also use a plain document or a notebook.
It doesn’t really matter where you write your topic ideas down, just make sure you put them where you can find them later.
More topic ideas will come to you randomly from emails, browsing social media, conversations with your friends, late at night, in the shower, when you’re making coffee. You need a place to quickly capture these ideas. I like the Reminders app because it’s simple and syncs between my iPhone and Macbook.
Consider dedicating 15-30 minutes every week or two to brainstorming topics. If you have a co-host, have a call every few weeks to discuss possible topics for future episodes.
If you’re wondering what your first episodes should be about, here are a couple questions that will help you get started:
Do this: Set aside an hour for brainstorming and write down every episode topic or idea that you can think of. Try to get at least ten, but if you think of more, keep going. You’ll need them later, trust me.