You already know where you want it right? We'd always advise drawing it out with sand (or a rolled-out hosepipe) on the lawn before you start digging.
Check that it looks good from all angles (including an upstairs window if you have one).
Check that you can get around it to trim hedges, paint fences or even wheel the bin out!
Avoid overhanging trees. Dropped leaves and needles will decompose and turn to sludge in the bottom of a pond.
Avoid boggy areas too. As ground water levels go up the liner can lift out of a pond in a boggy patch.
Ponds should have sun about half of the day if possible to help water plants to grow.
Can you bring power to the pond easily? This isn't so important if you want a wildlife pond but is needed for most pumps and filters that help keep fish healthy. Solar power is an option for small fountains but not for waterfalls and filters.