It depends on the products you want to sell. Niches are very different.
You can`t make a website and sell products in 10 different niches because you need targeted visitors.
Create a blog for a niche and post interesting and original articles, also create social media accounts to back up your blog.
The niche you choose is up to you, do some research and see what works best. For example I have been working on the brain supplements niche in the past months and I have had allot of success especially in Europe.