Brain Dump First
Write down a list of all the stuff that you're awesome at. No judgment, no editing, just brain dump. Ask your friends if you need to. You're probably taking some stuff for granted and are undoubtedly more awesome than you think you are.
Do Your Research
Is this profitable? Do people actually buy this thing? Go look on Google. Search your ideas and look at all the paid offerings that show up in the first page results. Who are they, what are they offering, and how much are they charging? Make a list. Do the same with Amazon. What's for sale and how many people are buying? We're just proving that people actually want what you do.
Feel Your Customer's Pain
How painful is this problem? If I break my arm, I don't give a shit how much it costs to get it re-set, put in a cast, and fixed. It's a huge pain point. How much people pay is directly related to how hard they're hurting. When you know who you're serving and how much they're hurting, you also know almost everything you need to about how to speak to them and serve them.
Scope the Competition
Can the people you want to serve actually pay you for what you do? If you have competitors, people who sell essentially the same thing, the answer is undoubtedly yes. You also have an opportunity to shake things up in that world and present a fresh alternative. Look at what your competitors are doing, what people are engaging with, and then think about how you'll do things better, do things differently, and add more value than your competitors are.
Listen and Adjust
If you're just getting started, you'll find your unfair advantage really fast. And when you dive headfirst into that and aren't set in your ways but are set on listening to what your audience wants, you can't help but grow fast. There's room for everyone, so don't let yourself believe that just because someone got there first, you can't make moves.
Finally, it's time to test. Experiment with EVERYTHING in the beginning. You have no audience yet, so you have nothing to lose. Once you start to have people follow and subscribe you, there will be certain expectations on you. Right now there are none. If you're consistent over time and adjust course as people tell you what they want more of, you can't lose. You never know what's going to work until you put things out there.
Test with free content to see what resonates - any feedback is good feedback, even no feedback. If people aren't engaging, it's probably not what they're looking for. Or, it's delivered in a way that doesn't speak to their pains. Test with pre-sales of a product you're building to get capital to reinvest or with a beta group of users who can figure it out along with you. There's no bigger mistake than thinking you know what your customers want without asking them or listening to what your testing tells you.