If you are completely new I suggest watching a tutorial. Otherwise some basic tips could be
if oscilloscope has multiple channels, make sure the channel you connected is the channel nel you see on the screen
make sure you did the ground connection. You need to connect both signal and ground tips if the probe
make sure voltage range is fine. If you use a large range in the screen, little amplitude you got may seem like a flat line.
calibrate your probe, if oscilloscope has that option. But an uncalibrated probe doesn’t explain flat line, it would explain distorted square wave, for example.
Use another probe. They can get broken. If it is broken all you will see will be the noise the cable pics up like an antenna.
separate the issue. Connect probe ends to a battery with k own voltage and see what you receive. If it doesn’t work, you know either probe or oscilloscope is wrong.
List can get longer really, but I think a tutorial and these can go a long way for the beginning.