If you are wanting to purchase cannabis in California you will need to be present in the state to get your card and then you can go to a dispensary or delivery service.
If you are asking how to get a card in Hawaii to take advantage of purchasing there you will need to look up the laws in your state, which is available here:
I hope this helps.
I am licensed in Hawaii and can help you in HI but as stated for CA, you must be physically present in CA to get a card. In HI we are still waiting 16 going on 17 years for our first dispensary. The first dispensary was to open in July this year but has again been put off till maybe 2017. Really discouraging for the people who are sick and use the cannabis medically because it is illegal to transport any cannabis into the state and most cannabis is not tested in HI.