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Your computer may be pre-installed with a default program (Windows Movie Maker or iVideo); if not, you should be able to download free video editing software from the Internet.  Make sure that the clip shows exactly what you need to prove your point. Whether you caught a sibling stealing from your room or a coworker stealing from your cubicle: the clip should start a few seconds before the crime and end a few seconds afterward. Save the relevant footage in a new video file, but do not delete the extra footage just yet. Sometimes evidence appears where we least expect it. You may find something interesting when you review the extra footage. If you plan to use this video to make any sort of legal or professional case, it is important to protect your evidence.  Consider emailing yourself a copy of the file. Even if someone goes to drastic measures to find and delete the footage from your computer, you will have a remote copy that you can download onto any computer with an internet connection. For the same reason: consider saving the footage onto a USB flash drive. Keep the drive in a safe place. Be tactful. Tell them that you know that they were sneaking around in your things, and ask them them not to do so again.  You do not need to bring up the hidden camera immediately. It may come in handy later on, if they continue to sneak around after you've asked them to stop. If they own up to their snooping ways when you confront them, then you don't need to show them the evidence. If the snooper knows that you've hidden a camera somewhere, and knows where you've hidden it, they may find a way to sneak around it. Don't reveal your trump card right away. Some people don't learn the first time they're caught. Be aware that depending on who you're dealing with, merely catching the snooper in the act once may not stop them from snooping in the future.  Consider leaving the camera running when you suspect that the snooper might strike again. It never hurts to be sure. If the problem continues, tell an authority figure. If you're dealing with a sneaky sibling, tell your parents. If you're dealing with a sneaky coworker, consider notifying your supervisor. If the snooping is of a criminal nature, consider contacting the police.
Use a video editing program to isolate the clip of the crime. Save an extra copy of the footage in a separate folder. Confront the snooper. Forgive, but don't forget.