How to make https request and encrypt POST parameters?

I have developed a Mobile application that connect to my Web Servie via RESTful API.

Now, I need to protect my API by someone who could sniff it via WireShark. I just need to hide my URL and POSTed parameters in some way...How can i do it?

My risk is that if "someone" get my API's Url and change ?user_id parameter of each request could act as another users, and do bad things...

(I don't use Oauth 2.0)