You should already know what products to get and you should have good experiences in your intended industry already.

Product selection methods at the startup stage are different from the growing stage or mature stage ...

It will be a hard time if you don't have the product knowledge and no industry exposure and experience. There is no single method that can answer your question and no simple approach can be described via simple writing to address your challenges.

I help 400+ companies to grow correctly. The most important point here is "correctly". Anyone can grow a company, but the company may not grow correctly and many people were misled by wrongful growth and eventually failed very badly. Therefore, my advice to you is not to take loose advice or choosing some product selecting methods to address your answer to avoid a disastrous outcome. This comes from a real business case where a client as me to help him to salvage $500,000 in slow-moving inventory due to select products using some well-known methods. Always view product selection as part of building a business model, never do it from an isolated viewpoint.

Hope you are clearer and if you like to find out how to build a Startup footprint for your business, feel free to contact me.