The iPad1 can only go up to iOS 5.1.1 - and it's a pretty simple upgrade. To do:
Connect the iPad to [the latest version of] iTunes. Before you do anything, I recommend you (1) right click on the iPad tab and select "transfer purchases." that'll update iTunes with all the apps (including any updated ones) that are on the iPad.
Then, (2) make a backup of the iPad. You will need this recent backup later on in the upgrade process.
With those done, on the summary page of iTunes, you should see a button titled "Upgrade." There might be one titled "Restore" instead if you've stopped iTunes from looking for iOS updates. doesn't matter - click whichever button is showing.
iTunes will start its magic and, after some prompts, you should be asked whether you want to restore as new or from a backup. Select the backup you just made and iTunes will then finish the upgrade process.
Once iTunes is finished, you should have an iPad1 running on iOS 5.1.1. Good luck and let us know how you get on.
Marilyn