Play without a keyboard instrument
When you don't have a keyboard instrument (yet) or can't make the connection at the moment, then there are options to use the iPad/iPhone/iPod as an accompaniment instrument on its own.
In Settings select MIDI Out is Soundfont player. Touch the "No soundfont selected" button and then the "Download Soundfont" button. You will go to the soundfont page in Safari. Download a soundfont and select "Copy to One Man Band". You will return to the soundfont list in OMB, touch the downloaded soundfont to activate it.
You must realise that playing a lot of instruments at the same time asks a lot of the processor. Older devices like the iPad2 will probably experience hickups and distortion. In that case you can try to avoid that by using only Main A variations and only single instrument voices like 001-piano etc.
There are two panels available for playing without keyboard, "Virtual Keyboard" and "Chords", but you can create your own panels if they are not convenient. In Virtual Keyboard the C-key chords (C, F, G, Am, Dm, Em) are in the top-centre, easily accessible with your left hand thumb while your fingers hold the device. Two octaves of notes are at the bottom, play them with your right hand fingers while your thumb holds the device.
To play only accompaniment (e.g. while you sing) use the Chords panel where all the major, minor and 7 chords are available.