Because this is quite a technical career, additional tertiary study will be required. You could look at study within information or computer science, or information technologies. Having some experience with security and also keeping an eye out around web or visual design may also help.
Unlike the other creative roles, there is not a ‘book’ or portfolio to build to demonstrate your early work. Get started through applying for graduate roles (like Summer of Tech), entry level tech or IT roles.
Mostly, you would need a CV and Cover Letter. You could connect with CEOs from agencies that you would like to be a part of, introduce yourself and see if there are projects which might have some opportunities for you to contribute.