how can I download excel to my ipad

You can't. Excel will not run on an iPad. You can purchase Numbers from the iTunes store and that will allow you to read and write Excel spreadsheets.

Excell is not compatible with the iPad itself. You can use, however, Numbers and that app should be able open excell files.

There is no Microsoft Excel Application as such for iPad. Though there are several alternatives in the App Store for spreadsheets including Microsoft's own Office Mobile for Office 365. Which includes mobile versions of Word, Excel and other Apps. The App does require a subscription to Microsoft's Office 365 Online Office service though.

https://itunes.apple.com/en/app/office-mobile-for-office-365/id541164041?mt=8

The only version of Microsoft Office for iOS is an iPhone app that also requires an Office 365 subscription. In addition to Numbers mentioned above, there are several MS Office compatible apps available at various price points in the app store (CloudOn, Documents to Go, Documents Unlimited, to name a few). In addition, there are numerous apps that will display MS Offcie files, but do not provide teh ability to edit.

You do need to be aware, however, that all of these will hav limited functionality compared to MS Office for Windows. Pivot tables are not available, for example. So depending on your specific needs, iOS may not be the platform for you for this type of activity.