I'm not really sure I understand exactly what you are trying to do, but the easiest thing would be to use the Android WebView
If you really want to do the logon stuff yourself then you have a bit of work to do. You will need an activity with text boxes for a username and password and a submit button. The submit button should connect to the websites logon cgi script. If you vist the websites logon page on your PC browser, right click it and select "View Page Source" you can see what the page does. You are looking for a HTML form, you need to read up on HTML GET and POST and then do whatever the form does in your code (see the WiktionarySimple
sample app for an example of HTTP GET from an Android app). Make sure you are using HTTPS if you are sending passwords.
I still think you would be better off using the websites API if it has one though. The WiktionarySimple app demonstrates this.
document.getElementById('leftColContainer').style.visibility = 'hidden';
The side bar will disappear. You can get it back by typing:
document.getElementById('leftColContainer').style.visibility = 'visible';
Read up on: