If you are renting, get the permission of the landlord or you might get charged for unauthorized modification considered to be damage. Some landlords might want to do the installation themselves.
Note that you generally buy the peephole and then follow the instructions for that peephole.
Some peepholes have a little cover on the inside that swings out of the way. Some peepholes give a sharp image at long distance focus and some require close focus. At age 74 I need 2.5 D reading glasses so I can tell the difference, but a young person might not detect any difference. Through some peepholes I get a sharper image with my readers on.
If when you say the door is without a hole, but you are allowed to drill one then it is a question of deciding the height and a suitable diameter.
If you cannot drill a hole then some sort of miniature camera will be needed...
When drilling a hole through the door, making sure you go through square is crucial and a tiny pilot hole then drilling from both sides gets a clean hole - at least in my experience.
Here is one possibility: Peephole on Amazon