How to Create Plugin that Runs on Post?

I have One Plugin which Creates Custom Posts, Now I have One Nested If Else Code I want to Check On Load of Every Custom Posts.

How to create said plugin, Guide Me with some scratch code as I am new in Plugin Creations.

$date1 = new DateTime($comdate);
$date2 = new DateTime($blogtime);

	if ($date1 > $date2) //date difference
	{

		$myset = $someintigervalue;

		if($myset <= $someintigervalue)
			{
				//Load Custom Post
			}
		else
			{
				//Display Message with Echo
			}

	}
	else
	{
		//Display Message with Echo
	}