In order to go lighter than your current tone you will have to remove your existing darker base. It is possible to go back to natural however it may take a few times to get the tone exactly right since it is hard to really know what to go by when only comparing to the roots. I would go in to see a specialist initially and once she's got you where you want to be ask her how you can maintain it yourself. This will save you a tone or damage and heartache as color correction can become a bit complex when trying to do it yourself.

I would recommend going to a stylist to do it or even going to a beauty supply and asking for assistants in recommending a color remover that would be best for you. Then I would say to use a deep conditioning treatment afterwards.