There's no easy way to describe it briefly, but I'll start with Cliffs Notes, described in more detail below:
--If your old point-and-shoot worked well under identical conditions, then it could just be that the 50D's ISO setting, which controls how sensitive to light the sensor is, is set to a really low value. I would first try to change it to Auto or High. If that doesn't work, manually set it to 400, try shooting, then bump up higher until the exposure works.
Set/Change ISO levels
You need to find the ISO control on the camera and see if it's set to a low number (200 or less). If it is, first try changing it to Auto or High, if it has a High setting. If this doesn't work, start bumping the ISO value up manually, one setting at a time (higher number = more sensitive, that is, better in low light). Note, though, that as you start to get into higher and higher settings, you'll get some noise in the image.
If Setting ISO Higher Doesn't Work
Do this (described below)
--Put camera in Program Mode
--Suppress flash, if necessary
--Retract lens all the way in, move closer to subject
Try is to see if it'll take the proper exposure by itself by putting it in Program mode (P): this may be a "P" on the circular dial on top, or in the menu somewhere. (Program chooses all settings for you -- it basically turns a very fancy camera into a point-and-shoot.)
Now in Program mode, if in your low light environment you do NOT want the flash, then you need to suppress it by finding the flash menu (or button) and force it off by choosing the icon that is most likely a circle and slash with a lightning bolt through it.
Now, if after putting it in Program and suppressing the flash the shots are still under-exposed, the next easiest thing to try is to back what I assume is a zoom lens all the way in and move the camera as close to the subject as possible. With the lens backed in, you get more light than when zoomed out, and the camera will work better with the available light.
Still Not Working?
If your shots are still not working, then, yes, at this point, put the camera in Manual mode, keep the lens backed all the way in, still as close to subject as possible, set the aperture, aka F-Stop, to the smallest number possible (this allows the most light in). Set shutter speed to something slow, like 1/30th of a second. If the shots don't work, go to 1/15th, try again...