I think you may need to start by determining or get a rough idea of what type of ecommerce business you want to start.
There are a lot of platforms out there but each one has its strengths and niche. Identifying the type of ecommerce business you want to get into or how you intend to sell your products will help you narrow down the list.
Again, how much the platform costs will also further narrow down the list as well as how easy (drag or drop, or writing code) using the platform is. Also things like how good the customer support might also factor in.
As you knock out the platforms that are not suitable, you will find you have a much shorter list of ecommerce platform to choose from and hopefully it will be easier to pick the most suitable one.
Hope this helps