I've built a couple of ponds and waterfalls. Its not cheap for the liners, pumps, wiring, etc... I'd also recommend putting motion sensors on a couple of sprinklers to protect your fish from *****, and herons. I've lost a ton of fish without them. I made my first pond back in Junior High for a project. Its a lot of fun, I think.
We recently finished building our new house on 5 acres. It was raw land and we left most of it natural, except where we needed to clean up our view. I have to build another Koi pond in the backyard, and A larger pond that I will plant with trout, bluegill, or both. The small pond is already dug, I'm just waiting for the weather to get a little nicer before I finish it. The bigger pond is currently under water. Probably 1/4 to 1/2 acre. It seems to gather and hold quite a bit and it stays about 9 months of the year. It has a small culvert that runs the excess off a hill. If I stop the culvert from draining, it would hold water all year. This August when I get back from salmon fishing in Canada, I'm going to rent an excavator and clean it up, and shape the pond. I suspect that I will hit a spring as there's really no reason for water to be there in the first place. I'll let it fill up over the first rains of the fall, and hopefully by this time next year, I'll test the water and be getting ready for fish!
There are several books on building ponds available. I even saw a couple at Home Depot. Good luck, and keep a picture record!