What do I need to know before starting an eCommerce business?

First of all you should know what kind of businness you wanna make. Is it entrepreneurship or existed company new goal ? According to it, there is few other things you have to focus. But both common things are

1. Who is your target group?
2. Do you use social media effectively to reach them ? 
3. How much hour can you take care of your e-commerce website? 
4. Will you be alone or with team ? 

These are just basic start questions.

Iıf you are entrepreneur, you have to choose your genre for what to sell. If it is a thing to sell like Clothes, Electronic devices etc.... you should use Instagram to take people attention with target focused hastags (#) at first noone would like to give their credit card to new created unknown e-commerce website cos. 

If you are doing this project for existed company, again social media will help you a lot. But things will be much easier for you to sell your things at there.

Both has technical needs : 

1. Accountant Application to follow your stocks
2. Nice brand name and website domain name
3. Better host to not give you problem
4. Quality e-commerce system. (After reach 100.000 $ income i think inhouse type software your only options) 

If there is further question please do not hestitate to contact me.