I have to log in on an HTTP website.

There is a login form which contains inputs for username and password and as hidden inputs the sessionId. I am creating an application in which I have to access resources which just can be accessed if you are logged in on this website, so I provide a username and password input in my application to log in.

I watched the HTTP requests now, and the HTTP POST request in which the login data is sent has the parameters password and username, so I could see my username and password in Fiddler non-encrypted, but I don't want to send my data unprotected.

If the parameters of an HTTP POST request can be seen by tools like Fiddler in the clear, does this mean that my data is sent without any encryption to the server? Or is there any kind of encryption that is done which just isn't visible to me?

Ordinary HTTP of all sorts is unencrypted.  If you want to protect your data, it has to be sent over HTTPS.