hole so you can see who is at your door
They sell peepholes, at most hardware,stores. Should have instructions,.
here's how to videos you can sort thru since you don't say what kind of door it is -metal?wood?https://www.youtube.com/results?search_query=DIY+installing+peep+hole+in+door
If you have the budget, I would recommend you try a video doorbell instead. We also have one and it definitely gives you peace of mind because it allows you to check what's happening in front of your door even if you're miles away (using your phone).