I have Lexmark 6570 printer. How can I hook up to ipad

Unfortunately, your printer isn't AirPrint compatible, so you can't print from your iPad.

The Lexmark 6570 does not appear on the list of AirPrint compatible printers. See this Apple Support document for details:

http://support.apple.com/kb/ht4356

You might still be able to print from the iPad to the Lexmark printer if you download and install an App like Print Central.

PrintCentral for iPad on the iTunes App Store

Another option might be Printopia. There is a free trial, so you will know in advance whether this will work or not. You need to have another Mac running on your network to be able to use Printopia.

Printopia - AirPrint to Any Printer - Print from iPad - Print from iPhone ...