What generation of iPad do you have? If you have the first generation (an iPad1), then you can only put iOS 5.1.1 on it now. If you have the second generation or higher iPad (iPad2 or above), the the only iOS you can put on that iPad is iOS 6.0.2.
Regardless which iPad you have, or which iOS you can install, the easiest way to upgrade your iPad is to connect your iPad to iTunes. After connecting it to iTunes, the first thing you need to do is make a back up (right click on the iPad line and select "Backup").
After the backup is made, on the main Summary page of iTunes, click the button that reads "Check for Updates" (it may already say that there is an update ready; just press that button if it's there).
iTunes will do its magic to upgrade your iPad. When prompted, elect to "restore from backup" (and use the backup you just made). Then, when it's all over, you'll end up with an iPad on the latest iOS it can run.
Good luck and let us know how you get on.
Marilyn