How do I file? What do I do? What do I need?

I’m going to try to explain this the best I can I apologize if it’s hard to understand. the beginning of the year I started doing product reviews. The companys or person sends me the money for the product I need for the product,then I order it and after I receive it and can test the product I give my honest opinion and write review for product. I don’t get paid for doing this but I get to keep the items I review.  Most of the items are junk and get thrown away but there has been a few that I’ve personally kept, some I’ve given away as gifts, and I have also sold some. 

 Some items have an inflated price when I order then after a few weeks they go down like here’s an example I order a $200 blender then after I order it they change the price down to $50  and keep it at $50. I bring this up because I seen somewhere I had to find the value of it if I sold it and then how much I sold it for. 

How should I file and what all receipts/proof should I get together? I’ll be getting a paper from PayPal can’t remember what it’s called rn but because I made over the amount I’ll get it. 

If you have any advice or anything on what I need to do or should start doing that would be great I’m young and dumb and completely forgot that doing this would affect taxes and stuff and I’m so nervous