Round door knobs
Round door knobs are very common they can be turned both up and down. So to open the door grab the door know and twist it in any direction. If the door knob is locked look at how to use lock picking section. After you twist pull, if that does not open then push the door.
Flat door knobs
Flat door knobs can be open in both directions. So grab it pull up or down. Then try pushing and pulling the door to see what way does the door opens. If the door knob does not turn then its locked. look at how to lock pick section.
Push to open doors
Push to open doors are made to able to open at all times from the inside and stay locked on the outside. To open the door all you have to push the metal part on the door and push. Then the door will open.
How to lock pick
To open locked doors you will have to learn how to lock pick. And do that watch this short video of the bassics here! after watching the video try to twist and pull and push on the door to open it.