Without knowing how much water you can capture and need to capture, this is a pretty tough question to answer.
A small 50 gallon system barely will hold enough water for a small garden but would be great for a few small flower pots. If you need something larger, you can consider a Rainwater pillow, a concrete catchment pond, or a host of other options.
I am not sure who or what is available on the island but a quick search for rainwater harvesting Hawaii should bring you to a few nursery stores or landscapers that could help you out by sizing it, getting it set up and teaching you how to use it.
For more information:
Read "I need advice about creating a bioswale or rain garden in my yard where overflow rainwater collects." a Q&A answered by Richard Heller.
Also, read this blog post by Sean Lintow.