Removing Doors and Windows
The following demolition steps are virtually the same for either a door or window:
Remove the interior casing around the unit
Remove all the nails holding the jambs to the framing (use a nail-puller or cut the nails with a reciprocating saw)
Outside, pull any nails securing the exterior casing (on wood-clad units)or nailing flanges (on aluminum or vinyl-clad units).
Pull the unit out of it's rough opening.
TIP: With heavy doors or large windows, remove the door or sashes from the jambs before trying to pull them out, especially if you're working alone.
Once the unit is gone, you can fill in the old rough opening. On a door, first nail a new bottom plate between the trimmer studs over the threshold. Cut and nail in a cripple stud every 16" between the header and the bottom plate. Cover the opening with sheathing and match the exterior siding. Cover the remaining interior wall with wallboard.
Removing Drywall & Plaster
The usual technique for removing plaster or wallboard is to smash it apart with a hammer and pry off the pieces individually. Other remodelers first use a reciprocating saw to vertically cut through the wall surface from floor to ceiling between the studs so the material comes off in larger pieces.
CAUTION: Be extremely careful when breaking into walls because water, waste, and electrical and/or gas lines are always found somewhere in the stud cavities of kitchen walls.
To save on finish work later on, determine how much of the wall you really need to expose. Find the nearest wall studs outside that area and remove only that much of the wall.
Taking walls apart is dirty and rough, so you should wear a dust mask, safety glasses and ear plugs. Cover the floor with plywood and plastic if you want to protect the surface, and have a trash can on hand for collecting debris.
All of your home's exterior walls and most of the interior walls were originally framed as load-bearing walls to help support the framing above them. If any of these wall studs are removed, they must be replaced by some other load-bearing member—like a header over a new window or door for example.
Remember to check with your local building officials beforehand if you're planning to change any framing. They'll review your plans, highlight any problems, and suggest solutions in addition to the needed permits and inspection schedule.
NOTE: Work that violates local building codes often invalidates your homeowner's insurance, along with possible health and safety dangers. Getting a permit and having work inspected is your assurance that the job is done properly.