After you made the func_door leave it selected and press N, to bring up the entity window. There are buttons in a "circle" at the bottom left corner to set angle. But of course you can enter it manually. Key: angle, value: what you want.
If you are new to mapping I encourage you to have a look on the resources thread. It is stickyfied. Best tutorial for beginners is Rich Diesal's tutorials
. And of course there is the forum's search function.