Category > Subcategory Display Menu

Hi,

I have been searching for this code all over wordpress forums and stackoverflow but didnt found a thing. I wish to create a menu on the post (single.php) sidebar that will list all categories and posts that fall under the parent category of the category of this post. So, for instance if I am on Post-3 page and it falls under Cat-2. And Cat-2 is a subcategory of Cat-1 then I want to list all subcategories of Cat-1 along with the posts they have. At the same time, I do not want to list the posts categorized under Cat-1.

I am very new to wordpress and though I found and tried a lot of functions and codes, none seem to work for me. Somebody please help me and save my day!

Thanks in advance,

VD