How to ship things you sell online?

Basically I have this app called Mercari which is like a buy-sell marketplace app. You can just basically take pics of clothes you don't need and want to sell and post them with a price, description yadda yadda. Well I've been buying off the app but now I want to try to sell some stuff I don't need anymore. The only thing is, I'm clueless when it comes to mailing things. You can choose how much you charge for shipping on the app and mercari also emails you a prepaid shipping label. 

So, do you print out the label and go to the post office with your item? Do you have to buy a box for it to go into? So if I sell a shirt with $6 shipping, that means the buyer already took care of all shipping costs right? So do I still have to buy something for it to be shipped in at the post office?? If anyone has sold stuff online and wants to walk me through this process I'd be really really greatful. I'm clueless 😫