function wstaw(myField, myValueStart, myValueEnd)
{
    //IE support
    if (document.selection)
    {
        myField.focus();
        sel = document.selection.createRange();
        sel.text = myValueStart+sel.text+myValueEnd;
    }
    //MOZILLA/NETSCAPE support
    else if (myField.selectionStart || myField.selectionStart == 0)
    {
        var startPos = myField.selectionStart;
        var endPos = myField.selectionEnd;
        myField.value = myField.value.substring(0, startPos)+ myValueStart+myField.value.substring(startPos,endPos)+myValueEnd+myField.value.substring(endPos, myField.value.length);
    }
    else
    {
        myField.value += myValue;
    }
}

<form name="FORMULARZ">
  <input type="button" onclick="wstaw(FORMULARZ.TRESC, '[start]', '[stop]')" value="wstaw" />
  <textarea name="TRESC"></textarea>
</form>

function nazwafunkcji() {
   var txtarea = document.getElementById("area");
  var selStart = txtarea.selectionStart;
   var selEnd = txtarea.selectionEnd;
document.getElementById("area").value=  
(txtarea.value).substring(0,selStart)+
   "[b]"+(txtarea.value).substring(selStart, selEnd)+"[/b]" +
   (txtarea.value).substring(selEnd,txtarea.value.length);
}

[HTML] pobierz, plaintext 
<textarea id="area" name="nazwa" rows="15" cols="50" ></textarea>
 <button onclick="nazwafunkcji();return false;">Pogrubiona</button>
[HTML] pobierz, plaintext