How do I advertise a product on the internet and sell it?

For one-offs ,there's nothing better than e-bay.

I think you mean a product that you are producing in quantity, and that you wish to sell. In that case, you need your own web-page which is effectively your electronic shop. Make sure it gets picked up by the search engines.

There are numerous organisations who will host your webpage, search for "webpage host".

Good luck

Alan pointed it out clearly.

Then you would most certainly have to advertise your online shop and add it to many directories. Online marketing is very different to offline marketing.