If you have a passcode to the screen lock that you've forgotten and your device is running iOS 6 or earlier, restore the device from the computer to which the device is synced. For information and instructions, see:
If that will not work, you'll need to put the device into Recovery Mode and then try the Restore again:
If that still doesn't work, as a last resort try DFU mode:
If your device is running iOS 7 and you set up Find My iPhone (iPad/iPod), however, then it has the Activation Lock on it and you'll have to erase the device through iCloud first:
Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.