You'll probably want to tell us how you visit the site...as using a phone vs a laptop may mean doing things slightly differently.
In general, you visit the area of the Exchange appropriate for the item you wish to sell, such as folding knives, custom knives, a particular brand etc.
Then you would open a new thread and put in the particulars.
You can upload images directly from your device by using the image icon which looks like a postcard, or you can link from an offsite album such as imgur or similar.
If you are having a specific problem, try to provide detail for someone to reply to.