The Canon EOS 50D is a DSLR (Digital single-lens reflex camera). This means that you will NOT see a digital live image in normal operation unless you press the shutter, in which case you see a review of the shot taken. Instead you have an optical viewfinder.
The camera operates by having a mirror redirect the light to the viewfinder instead of the sensor. As it is not hitting the sensor, you cannot get a digital live image on the back screen. This kind of operation would need a mirrorless camera.
However, you can emulate this by going to the live-view mode. This flips the mirror out of the way, so you can use the sensor to record a live image. Please note that this will seriously cripple your autofocus. And you cannot use the optical viewfinder during live view.
Xiota already gave you the link for enabling live-view: Enable Live View on EOS 50D