Before you start an online business, you need to do some thinking or planning, research and data gathering. You need to have a good planning if you want to involve in ecommerce, able to survive long term, and most importantly making money. A good planning will also ensure you will not get a lot of surprises in later stages, such as hidden cost in shipping overseas, payment cycle too long from certain marketplaces thus affecting your cash flow, shipping time is too long from your supplier etc.
Assuming you going to start from zero, here are some important things for your consideration. All these questions are interrelated.