First time writer

I hope this doesn't sound arrogant; I know I'm an amateur.

I’ve written the first chapter of a light novel and am looking for artists on commission sites to create artwork for it.

How do intellectual property rights work regarding my writing?

I know the chances of someone wanting to buy and adapt it are slim, but I don't like doing things only to regret not having done my research beforehand.
Where should I post it, and how do I ensure I retain the rights to sell or license what I’ve written?