You need a power source, power supply or battery
A sounder, alarm siren…
And a detector device
I have used a trip wire for very many applications; it is very low cost works very well.
Go to an automotive store, get an ignition breaker contacts, or garage and ask for old breaker points, they usually have lots.
You connect everything in series.
you place a wire across the room with a piece of paper between the contacts
when someone trips the wire, the alarm will sound
Hope this properly answers your question