<script src="http://code.jquery.com/jquery-latest.min.js" type="text/javascript"></script>
<script>
$(document).ready(function(e) {
    $('#metin').on('keyup',function(){
        var yazilan = $(this).val();
      	yeni = yazilan.replace('görmek','bakmak');
		$("#metin").val(yeni);
        });
});
</script>
<form action="" method="post">
	<textarea name="metin" id="metin" rows="10" cols="80"></textarea>
	<input name="gonder" value="Gönder" type="submit">
</form>