It depends on what you like and who is going with you.

You only have time to go one island.

Since this is your first time, you normally should go Oahu (Honolulu).

Waikiki Beach, Pearl Harbor, Hanauma Bay, Polynesian Culture Center and so on.
This link may help you:
http://bbs.wenxuecity.com/hawaii/264257.html

If you really like beach and want less people, go Maui. You can enjoy beauiful beachs, do snorking, drive to 'Road to Hana' and visis 'Haleakala National Park'.

Xiong