If you can only get access to a pool, train by treading water for long periods of time, and holding your breath under water for long periods of time.
To get the most for your time and money you’ll want to pick the right spot at the right time of day every time you go out.
By the end of this period you’ll start to realize the stiffness, flat nose, and buoyancy of a foam board is difficult to use on green waves larger than 2 ft.
That’s also an option, but the world of fun boards is so vast, with so many shapes and sizes, that you can spend a lot of time getting to decide your style of surfing, and what board best fits that style.