You should generally mark up your content semantically and then style it.
It looks like you have five main areas:
- messages
- user list
- controls
- input
- chatrooms
Let's make some markup:
controls
And a little bit of style:
.left{
clear:both;
}
.left div{
float: left;
}
#controls{
clear: both;
}
And you should be able to complete it from here. The most important part of most CSS layouts is proper floating.