I have been a student of the Hawaiian Language for many years.  It has a lovely melodious sound and a grammatical style that appeals to me.  Online Language lessons are available from the University of Hawaii at Hilo and a Kamahamaha Schools Language website as well.  Aha Punana Leo has a wonderful site to assist in the study the language and culture.

 Here are some other links for basic Hawaiian language info.  

Basic Hawaian - a great site to learn more about the language 

Kualono - University of Hawai`i at Hilo 

Hawaiian Dictionary - Search for a Hawaiian name or definition.

If you want to study the Hawaiian language, you will need a good dictionary; this is the definitive Hawaiian Dictionary: revised and enlarged edition, written by Mary Kawena Pukui and Samual H. Elbert, published by University of Hawaii Press, Honolulu, HI. 1986

This is a more recent and graphic book, but it is less comprehensive: Illustrated Hawaiian Dictionary by Kahikahealani Wight, illustrated by Robin Yoki Racoma, published by Bess Press, Honolulu, HI 1997.

In addition to a good dictionary, you will need Place Names of Hawaii, written by Mary Kawena Pukui, Samuel H. Elbert, and Esther T. Mookini.  

There have been many books written in recent years, and several videos and audios published as well.  I will be adding more recommendations.