@HayyatAli
I’ve already built four or five magic mirrors now. I always proceed the same way:

Buying the Raspberry Pi, as @nischi has already posted Buy a monitor the size you want to install later.
Please make sure that you get a monitor that switches to sleep mode without a signal on HDMI and does not switch to a completely green or blue screen for minutes before :-) Installation of Raspian Installing Magic Mirror
(just follow the hint from @nischi here) Installation of additional components such as motion detectors or a button to reboot the Raspberry then run the whole thing without wooden frame and mirror for a few days/weeks to detect bugs, try modules, detect incompatibilities, etc. Construction of a frame from e.g. wood
Then take measurements for the mirror glass. Purchase of a spy mirror Assembly of components

Here you can find a building instruction for my big mirror in the bathroom. I mounted an aluminium plate behind the spy mirror, which gives the whole thing stability, holds the monitor elegantly and ensures that no light falls through the spy mirror from behind.

Greetings
Sepp