How to secure post request data?

So I have a website with a register and a login forms. They send post requests to the server. So... the data isn't shown in the url of the browser, but still of one can see the post request's body he/she can get the valuable information.

Is there a way to secure this information, and should I secure it? It seems that this is an easy way to hack the website and should be secured. I don't know how though.