if you want to start VERY basic... just choose the wall that you want to put the door in, create a rectangular brush that is wide enough to go all the way through the wall. Place it in the wall, right click and press carve note carving is bad and you should get out of the habbit once ur more comfortable with hammer
Now, fit the door so its a bit thinner, right click, press 'Tie To Entity' then choose func_door. Choose the direction you want it to go in and viola