<script type="text/javascript">
function insert_tags(el, tagstart, tagend){
el=document.getElementById(el);
if (el.setSelectionRange) {
el.value = el.value.substring(0,el.selectionStart) + tagstart + el.value.substring(el.selectionStart,el.selectionEnd) + tagend + el.value.substring(el.selectionEnd,el.value.length)
}else{
el.focus();
var sel = document.selection.createRange();
sel.text = tagstart + sel.text + tagend;
}
}
</script>

Example:
<button onclick=" insert_tags('bbc', ' ', ' ' ); ">Bold</button> <button onclick=" insert_tags('bbc', ' ', ' ' ); ">Italic</button> <button onclick=" insert_tags('bbc', ' ', ' ' ); ">Underline</button> <br />
<textarea id="bbc" style=" width: 400px; height: 300px;"></textarea>