First off, choose your niche. You need to find a starting point for your business, and an idea of what you want to sell. Maybe, you can also start with this
guide on dropshipping for beginners. Even though it's important to follow your heart when you build a company, you also don't need to forget about revenue and customer needs. In the ideal world with pink unicorns, your store will combine all together: your passion, profit, and users' requests. And although we live in a real world, we would like to highlight that it is worth striving to balance these three components of any business.
If you struggle with finding what is on demand right now, you can always check
Google Trends to see what people look for and what problems they have.