Also go to your local pond supplyer, they will have many books on ponds, and ideas. The will probably also try to sell you their services so make sure to make it clear you want to DIY. Most pond stores are very helpful to DIYers and will provide you with good advice.
I have build about 6 ponds and i can tell you a few things, the first is that most people wish their pond was bigger after they built it so make sure it is as large as you can handle/afford. Remember you dont need lots of space for a large pond, you can make a 5 x 5 x 5 pond you have about 940 gallons. I recomend a minimum of 1500 gallons for a pond. Im assuming you want a pond specificly for your trutles so you probably want an area that is fairly shallow where they can go and get on a rock or a plate of sand or what have you. You also want a deeper area where they can escape from predators and get out of the sun if needed.
The rule of thumb is you want your pond 70% covered with plants, so do research on what kind of plants you want and what works in your climate. Usually a few healthy hardy lillies, some cattail, bull rush, anacharis do very well and provide good shade and hiding.
Good water flow and filtration is a must, although trtles are adapted to live in mucky ponds the ponds have a natural balance that your home pond will have to but will need a bit more help. A good filtration system is the best investment you can buy.
So what you need to do is figure out what is the maximum acceptable size of my pond, how much am I willing to spend, what kind of life might I want in it, what plants do I want and what plants work in my climate.
I am a novice computer imagery artist and I would love to make you an image of your pond ideas and then you can visualise them better. Send some drawings you might like that fit in your pond etc. Here is an example of what i can make pretty easily, it is a natural mountain pond but you will get the idea=
http://www.caedes.net/Zephir.cgi?lib=Ca ... 071495.jpg