How do I estimate what I need for data usage?

Explain what you want to do with the iPad?

The iPad primarily uses wifi for connection. If no wifi is available, then you can use cellular and you need a data plan. Did you get an iPad with 3/4G cellular?

 Cheers, Tom 😉

Most carriers have their own data calculator, for instance AT&T has this one:

http://www.att.com/att/datacalculator/index.html#fbid=NN9qiPJhTAr