Indexing for geographic objects?

I'm building a new geo type and would like to index it. I have heard about
RTREE and boundary box but I'm clueless for the moment about the
implementation....

I have tried to look into PG source code to find the location where the
indexing is done of current line object is done, but couldn't pin point where
is the code and how it works.

I would greatly appreciate if someone could guide me through the methodology to
build an index for a custom type or point me to some readings where the
algorithm is explained (web, book, etc...). I plan to use 3D geographical
objects...

Take this request as a newbie request, because I have never done database
indexing, not because I haven't done programming.

Please reply directly to my e-mail address
Thanks a lot.

franck@sopac.org