<script type="text/javascript">
$(document).ready(function() {
    $(".ship_to_checkbox").change(function() {                
        var ch = this.checked;      
        if (this.checked) {
            $("#mj").prop('disabled', false);
        } else {
            $('#mj').prop('disabled', true);
        }
    });
});
});
</script>

here is my checkbox code

<section>
    <input type="checkbox" class="ship_to_checkbox" 
            name="ship_to_checkbox" 
            value="Yes" {if $ship_check_result['ship_check']=="Yes"} checked="checked" : ""{/if}>
    <label style="font-weight: 400;">Ship to</label>
</section>