I need quick help on custom story creating!

[MrPur3d]

So i started to do custom story yesterday. But i never done a map before so i dont know what to do. I got some few questions for you. Please give detail with your answer.
1) How do i make a key open a door or cabinet i want?
2) How do i test my custom map in-game?
3) How do i create notes and write things in it?

Also, can you show me a basic, easy tutorial about creating a map?
I want to do something that there are lots of enemies and you need to close doors, jump over barricades, (creature jumps on you when you open a door, cabinet)
Thanks!