ajax with drupal

hi
In my site i am using the category module. With this i have made two different content type category i.e

category :
Technology (This is an input data)
Sub Category:
PHP (This is an input data)

The sub category input data is dependable to category cos i have managed the field with node referenced.

I have even created a separate content type called suggest tag. For this i even created a different node tpl for the customizing it. In this suggest tag node tpl page i have manually written a code (custom module is created) to create a form field.. The first field (selection list) i have created is category field and i even wrote a code to get the data to this selection list from the data base.. is working.

Now in the second selection list i.e sub category field i would like to get the data for the list depending upon the category. When the above selection list is chosen the subcategory data should be appear automatically..

so i tried with ajax.. but its really confusing .. so if u can help with ur suggestion.. it will be an act of gud sir....

looking forward