Here are some links you can start with:
This page, if read
carefully, explains how atolls are formed.
This page
has a good photo with some useful explanations
This page has a introduction
to atolls and also deals with some of the biodiversity of atolls.
Read this page
to understand what is likely going to happen to atolls and other low lying
areas as a result of climate change.
Click here
to see a fabulous 'Flash' movie on how Coral Atolls are created.