<form>
Тег 1<input type="checkbox" name="tag1" value = "Тег1"><br>
Тег 2<input type="checkbox" name="tag2" value = "Тег2"><br>
Тег 3<input type="checkbox" name="tag3" value = "Тег3"><br>
<input type="submit" value="Отправить">
</form>
<script>
document.querySelector('form').onsubmit = function(e){
e.preventDefault();
var data = "cat=1&tag=";
var boxes = document.querySelectorAll('input[type="checkbox"]:checked');
for(var i = 0; i < boxes.length; i++){
data += (i==0?"":"+") + boxes[i].value;
}
alert(data); // отправка GET
}
</script>