Apple's knowledge base is currently down, otherwise I would link you to a support article detailing the steps.
Restoring your iPad will involve wiping your device of all data, lets backup your iPad before attempting a restore:
Open iTunes, plug in your iPad, on the sidebar (left side) your iPad should appear, right click (or option click on a Mac) on your iPad and select backup. A progress bar will appear indicating your backup progress. Upon finishing, right click your iPad on the sidebar again and select "transfer purchases" to transfer your apps and puchased content to your computer.
Once that finshes select restore from iPad summary screen. iTunes will first download the operating system software from Apple severs, prep your iPad for restore which will erase all data on the device, then reinstall the operating system. Upon completion your iPad will reboot, on your computer you will be prompted to select your backup. This will restore your data back to your iPad.
This process can take a while (30 min+) depending on how much data you have on your iPad.