The chances are this is a hardware issue. If that's the case, it's probably not worth spending money getting it repaired, tbh.
However, there are a couple of things you can do to check it's not software. First of all, go into Settings|Apps and find Camera (it may be under "All" if there's a tab for that). Force stop it, then clear data and see if that helps. If it doesn't, the next stage would be a factory reset of the whole phone. This will put it back to its out of the box state and wipe all your data, so make sure your contacts and calendar and fully sync'd and that your content (photos, music etc) are backed up. If you want to keep SMS, there are apps - eg SMSBackup+ that will do that.
Good luck!
Edited to add: if you have an external sd card, remove it before resetting as the process sometimes formats the card.
