People who are used to modern automatic photography assume that using a classic camera is just as automatic. That's not the case.
You need to be careful with exposure and you need to be careful with focus too of course. As you can see, it's quite easy to waste a lot of money on film if you are not careful about what you are doing.
The ISO setting on the camera should match the ISO setting of the film that you have loaded in the camera. This allows the camera to "know" what exposure settings to use, depending on how sensitive or not your film is to light. However, with old cameras, the light metering system may not be very reliable to begin with. (It's also possible that there can be mechanical faults with the camera too.)
My first advice here I think would be:
Learn about the "exposure triangle", and what shutter speeds and aperture settings you should expect to be using. (You also need to ensure that your shutter speeds are not so slow as to introduce camera shake to your photos.)
See if you can download a light meter app for your phone. This will not be as good as a real dedicated light meter, but it might be better than the light meter built into your camera. Use the app to measure the light, and use the camera in manual mode, setting the corresponding exposure settings yourself.
Also, do ensure you read the manual for your camera.
I owned a second hand Ricoh Autohalf, I'm new to film camera and before this I washed a few roll of my films without knowing that my camera ISO setting should set according the the film ISO. It all went overexposed / underexposed. After consulting with the seller, I finally set the right ISO (on the camera) according to the ISO (on film). But it was half way through a film. After getting the washed film, it all went super overexposed and blurry.
