How to start selling online

Hi!!! I have no experience selling things online. Im tempted to sell on ebay/amazon but I dont know what I need to do things rights. Ok so you post your things on the site but aside from that… how do you know someone bought something? How do you get the info from the customer to ship out? Do i need something to make the label for the packaging or do they do it when you going to ship it? How do you know how much shipping is for that customer for items? When I get things, they are in different packaging from either the site im ordering from or just plain packaging so am I supposed to get the packaging or do the site provide their own packaging? I dont know if there is something else im missing but if you know, I would greatly appreciate all your help, tips. Thank you so much