First, you need to understand that you have two documents: The frame and the container (which contains the frame).
The main obstacle with manipulating the frame from the container is that the frame loads asynchronously. You can't simply access it any time, you must know when it has finished loading. So you need a trick. The usual solution is to use window.parent in the frame to get "up" (into the document which contains the iframe tag).
Now you can call any method in the container document. This method can manipulate the frame (for example call some JavaScript in the frame with the parameters you need). To know when to call the method, you have two options:
Call it from body.onload of the frame.
Put a script element as the last thing into the HTML content of the frame where you call the method of the container (left as an exercise for the reader).
So the frame looks like this:
<script>
function init() { window.parent.setUpFrame(); return true; }
function yourMethod(arg) { ... }
</script>
<body onload="init();">...</body>
And the container like this:
<script>
function setUpFrame() {
var frame = window.frames['frame-id'];
frame.yourMethod('hello');
}
</script>
<body><iframe name="frame-id" src="..."></iframe></body>