Validating Forms Input

I am looking for a little help validating a forms input. On the form are key
items which I do not wish to be zero length.

I have wrote the following function:

<script language=javascript>
function ValidateLength(oid, min, max)
{
var txt = document.getElementById(oid);
var length = txt.value.length;
if(length == 0)
{
alert("You must enter your reference, Company Name and Contact Name");
}
}
</script>

However their are still a few issues I would like to work out.

I all it with my forms sumbit event with the following:

<input type="submit" name="Submit" onclick="ValidateLength('txtRef',0,100)"
value="Submit Details">

However that will only check the length of the item txtRef, how can I check
more textbox values? Also how can I stop the form posting if the alert is raised?