Ren here from the Shopify support team, I'd like to help point you in the right direction for selecting products to sell.
My number one piece of advice would be to ultimately choose a product that you are passionate about or some way ties in to something you are passionate about. It's going to be a lot easier to sell something you believe in! Think about whether you want to self-produce or outsource the production or whether you want to utilize a drop shipping company.
We have a comprehensive guide available for free called How to Find a Product to Sell Online. There's 14 chapters of valuable information so this would be a great starting point to determining the right product for you. If you are leaning towards the drop shipping route, Oberlo has a helpful What to Sell guide complete with video to set you on the right path.
Finally, if you're looking for a shorter read just to get you inspired, consider the 12 Strategies for Finding Your First Profitable Product.
Let me know how you get on with that, and we can chat about any questions that come up!