Quick note- you will need "wooden planks" get wood from tree go to crafting table to make the wood from the tree into wooden planks.Ready to start! This is like a really easy thing to do, but in case your a beginner- this is how to make a secret trap door.

Okay so build a room, house, ditch, tunnel or whatever you want to be the location of the secret passage. Then this is, surprisingly, what some people "don't get" you have to DIG downwards at any point or place in your chosen location (make sure you have ladders with you) and once you decide to stop (don't go too far or you'll fall through the centre of the earth) you can make your secret room or area then you can start to put your ladders up the tunnel as you climb up and up to where you first dug. Now go to(or make) a "crafting table" and use some wood (thats why the note on how to get the wood is handy) to make a trap door place your trap door down (for better ability to access) the oppressive side of the ladders you put down your secret passage is complete. The only problem is.... It is visible to any player!!!

So I would recommend placing your bed or crafting table over the top that way it is safe from other players. But don't put a chest there. You ask why? Because chests keep items of yours in them and to another player that is what they want- your items and they might well take everything from your chest and once they've done that mine and steel your chest and- ohhh! They think they're clever do they? Now I know where they have been hiding! And that's your secret passage way and hide out blow!!

So watch out and be a clever crafter.