How do I upload images

Hello there! Please I need help on this, how do I upload images on my post, its killing me. I used the insert/edit image icon it didn’t work b/c it requested for the image URL and I don’t have but I have the image on my document. Also I try use the set featured image yet it didn’t work.Please I really need help on this. of what use will it be if I can’t use image to send my message across?.