Don't worry, that's easy. All you need is 3 events and an item!
Here's how: Create the item. Call it "Key" (for example).
Now create the door. That's a simple event with a door graphic. Then give a Conditional branch. Choose tab 4 and click the circle beside "Item" and choose the "Key". Then you can teleport the player or whatever happens when the door opens.
For example: If the player doesn't have a key, then when the player presses space in front of the door, it'll say something like; "You need a key to open this door.". Then when the player get's the key, it'll change it's graphic to an open door, and the player will be teleported through. Then change the graphic to a closed door again! Easy as that!