RESTful - when should I use POST and GET?

This is my WCF service, where user can find message for him. Simple:

    [OperationContract]
    [WebGet(UriTemplate = "/GetMessages/{UserGLKNumber}/{UserPassword}/{SessionToken}")]
    Messages GetMessages(string SessionToken, string UserPassword, string UserGLKNumber);

I have concerns about that line: {UserGLKNumber}/{UserPassword}/{SessionToken}

I have to authenticate user, before he get that messages. But with GET method, I cannot send objects, like in POST.

Is it consistent with REST pattern?

Please, clear up my doubts.