If you're talking about phpBB2 forums, you can download phpBB2 mods, hacks, skins, avatars for free from their Web site. They supply it all as archives and with installaion instructions. They've support forums also.

basically i want to know where exactly u want to make changes, i mean if want to add something on first page u need to modify the index file. or supporting .tpl file wherein u can add some code according to ur need.

Editing *.tpl files means affecting design and layout.
PHP files are also to be edited.

if u know basic html very well then u can edit the tpl file also its just an different extension given to html file which is being called in another html/php file. rest depends on u how do handle the codeing....