Category and sub category search

Hi,

wonder if someone can help me or point me to a plugin that do the following:

I need to add a category select drop down and a sub category select drop down. When the visitor selects a category, it should load it’s sub categories in the next select input (ajax). And the visitor can search for posts within that sub category.

I tried a few plugins but they don’t function as I need. I am not very familiar with php so some pointers would be great.

Thanks