There are various niches from which you can choose from. Why don't you go first create a list of 40 to 50 items which you think will be sellable. Then please go to Google and type in a product and see who and how many are selling. You can join discussions to know how the sellers and customers are reviewing etc and then decide. Yes it is a lengthy process, but hey you have to do that job and it is not fair on anyone saying this niche is the best for you.
I hope i answered your question so that you can start n your business journey.
You may feel frustrated when things do not go according to what you want but be strong and only with committment you can go forward and take on the world. My email is firstname.lastname@example.org if you want to discuss further...
My ecommerce site http://uniquefashionsonline.com (with shopify) will be starting shortly...
And the information site and affiliate landing pages of http://uniquesecrets.com, too will be launched soon.
I have although started promoting these two offers which are landing pages with a free gift. Perhaps this would also give you ideas.
regards Manoj Pillai