Can't make a sensible reply without knowing type of structure in which you wish to make the doorway. Briefly, if it is to go in an outside wall, this will be structural and you need to be very, very, careful. All the masonry above your proposed opening has to be supported before you start. This is usually done by making an opening about a foot wide above the height of the new opening and inserting a short beam though this, supported on either side, by "Acrow" props. The wall below this is then opened up to insert a lintel, which must be allowed to set, before any more work is done.
If of course the door is to go in to an internal, plasterboard wall, you just cut it out where you want, preferably between vertical "studs".