Try sell the most popular things in the world, thr indemand thigs, Like cellphone.You can have a great profit in it. But if you sell make sure that it is lesser than the mall price, The cheaper the more customer comes.
First of all you should do your homework. If you are planing to sell something online, target the audience/ customers and their likes and dislikes. Also if you can sell anything cheaper than what it is at the current market, the more likely are the chances you will get more customers.
The key is to gain trust and rest its all easy afterwards.
I agree with you, you must set a plan first and do some research regarding your targeted customers. The competition are already tough when it comes to online shop as there are a lot of online shops that being trusted by a lot of people already. Think about how you are going to get their customers and what you need to do to get their attention.