Q: How do I download software on my ipad

What type of software ? The only place to get apps for your iPad is the App Store app (and the iTunes app store on your computer's iTunes).

If you mean to update the iOS version that you have installed on it , then if you have iOS 4 as your tagline suggests then you can only update via your computer's iTunes.

To update the iOS version, connect the iPad to your computer's iTunes and copy any purchases off the iPad to your computer via File > Devices > Transfer Purchases. You may also want to copy photos and any important documents off the iPad as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPad via File > Devices > Back Up.

Then start the update by selecting the iPad on the left-hand sidebar, and on the iPad 'device's' Summary tab clicking the Check For Updates button

Updating to iOS 5+ : http://support.apple.com/kb/HT4972

You can only update a device to the highest iOS version that it supports, which for a first gen iPad is iOS 5.1.1, and for other iPads is 7.0.4

the only "software" that the ipad can use comes from the app store.

Peace, Clyde

Use the App Store app on your iPad.

Create an App Store account if you don't have one.

Read the Apps Store FAQ.