Related category/subcategory dropdowns in magento custom module -

i have created custom module magento admin.there form have 2 drop-downs(one category , second subcategory).how can relate theme.i want sub categories in second drop-down on change of first dropdown(state,city like).how can in magento using jquery/ajax.

here giving idea how sub category list ajax call. should have try self call ajax on change event of main category list , below code generate sub category list.

<select id="category" class="myinput-text required-entry widthinput" name="category"> <?php   $parentid=5; // parent id want sub category   $categories=explode(',',mage::getmodel('catalog/category')->load($parentid)->getchildren());   foreach($categories $cat){       $category=mage::getmodel('catalog/category')->load($cat); ?>    <option value="<?php echo $category->getid();?>"><?php echo $category->getname();?></option> <?php } ?> </select> 

hope sure you.