You need to give this a lot of thought.
Why do you want trout in the pond - you're likely to need a licence to stock them, and they require very good water quality and high oxygen levels - either a flowing input from a clean stream, or a good clean spring supply. They won't survive long in a muddy pond, and especially a lined pond with no inflow.
You'll also need something bigger than a mini-digger to create the pond. To be honest you need a pretty big pond/small lake to keep trout. It would be cheaper to go out and buy the fish from a fishmongers.
However even a small pond, that dries up every now and again can have a high conservation value - but don't put fish in them, and keep the ducks away as they will enrich the pond and it will become a green soup. If you want to create a wildlife pond, the Pond Conservation website is very good. The key points are to make it nice and uneven, large amount of edge, and plenty of shallows, and avoid islands! Leave the pond to colonise naturally - don't be tempted to introduce plants as they may bring nasty invasives in with them.
Planning - this varies from council to council, but for a small pond away from sight - I wouldn't bother
.