There are notes available from apple on the best settings to use (on the mac version of Keynote) if the slides are going to be presented via an iPad – http://support.apple.com/kb/HT4114

The general support pages for Keynote for iPad are here

The new iPads use the lightning connector, which can be connected to HDMI with an cable add on – available here

Be careful using the lighning adaptor (which allows old devices to work) as video out isn’t automatically supported – a new video out cable (matching the ‘plug’ of the device will almost certainly be required). There are all sorts of scaling issues with the new lightning connector (which may not effect slideshows, but if you are displaying apps with graphics there may be problems) – discussed in a recent blog post which has links to more technical information.