There are several things we'd need to know to help figure out what's going on with your memory card. While 50 images filling a gig card seems like a small amount, it's very possible with the right camera that this is an accurate behavior.
The first question is what kind of camera are you using and what format/quality are you saving your photos in? If your camera has the capability to save is RAW+JPEG (this creates both a RAW image and a JPG image for every photo you take), it's intirely possible your card is full.
If, after looking at how you're saving pictures, you determine that you shouldn't have a full card, then I'd think the next question would be, did you format the card in your camera before you used it the first time? I've never actually seen a problem where a card was not properly formatted in the camera, but I've read that this can be a problem.
If you just bought the memory card, plopped it in your camera and started shooting, it might be worth saving off the pictures you've taken and try formatting the card in your camera. Keep in mind that the format process will wipe out everything that exists on the card, so be sure anything you want on the card is removed before you do this.
The only other thing I can think of, at this point, is to check the options on your camera for where it saves pictures. From what I've seen, the options vary from camera to camera if the camera has internal memory. I suppose it's possible your camera has internal memory of say 50mb and the camera is still setup to save pictures to that location. It's possible it won't automatically over flow into the memory card you added. I'd check those options and make sure the camera is indeed writing to the memory card.
All of these options should be in the setup menu on your camera. If you're new to the digicam world, and are having a hard time locating the settings, post up your camera brand and model. I'm sure someone here can chime in and help you find the settings.
Good luck. I hope this helps,
Jeff