Creating Plugin From PHP Application

Hello,

I’m trying to turn a PHP application I have into a WordPress plugin. The app basically allows people to sign up then order stuff for free.

My question is, how can I call a PHP function when a user clicks a button or link. Say for example they click a register link, how can I make it take them to the register function. Here is a basic example of what I’m on about.

//WordPress Hooks
add_shortcode('repeat_module', 'run_bc');

function run_bc() {
	?>Click here to register.

	   <?php
	}
	?><?php
}

function bc_register() {
	?><p>Register here</p><?php
}
?>

Many thanks in advance