Ideally, the keyboard should be at a height so that your forearm is horizontal, i.e. parallel to the ground. Usually this would mean that the keyboard is a few inches higher than your knee when sat on a chair, e.g. a kitchen chair.
The main thing is to feel comfortable when sat at the keyboard.
You can get height adjustable piano stools (approx £80) and adjustable keyboard stands, which enable you to try different set up positions.
For more help with this, I recommend students in the UK to contact Chris or David at
www.epianos.co.uk/decplay.