how to use config to enable or disable public function

title says it all

Enable or disable public function? What do you mean? Really, I don't understand...

A public function is a class method, part of a class in object-oriented programming. It cannot be "enabled"/"disabled", because that doesn't make sense at all.
I'm assuming that you are referring to a public Event function in a Listener class passed into PluginManager::registerEvents(). It is called an event handler in a listener class.
Simply add

PHP:

if($this->isThisHandlerDisabled) return;

in the first line of your event handler function.

how to make config file step by step

Method 1:

PHP:

$cfg = $this->getConfig();
$cfg->get("Whatever"); //Get something in config
$cfg->set("Whatever", $value); //Set something in config

Method 2:

PHP:

use pocketmine\utils\Config;

public $cfg; //Make the property

$this->cfg = new Config($this->getDataFolder() . "config.yml", Config::YAML, array());

Instead of asking and looking for code, you should instead learn PHP and at least understand what a function is.

You should add

PHP:

$cfg->save();

as it otherwise wouldn't save the config.