The position of a pond in your garden is important to its future health. Firstly, it needs to be clearly visible from where you most often sit in the garden or from your house. It needs to be in a sunny position because the warmth from the sun will encourage plant growth and activity of the fish. Bear in mind that over half of the water surface needs to be shaded by aquatic plants, if in full sun, such as lilies so that the water is warmed, but not in the direct sun. Try to avoid placing your pond under overhanging trees or bushes, because their leaves falling into the water will create more maintenance on your part. 
 
Now think about the size and shape of the pond that is possible in this spot and the water features - fountains or waterfalls - you would like. You will need to get electricity to your pond. If you plan a waterfall, is your site naturally sloping, or will you need to create a rockery from the soil you dig from the pond? What will this mound back up to? Can this mound be used to hide a pond filter? The pond must be sufficiently large to take your planned waterfall without causing continuous turbulence in the water.
Thinking things out in this way is important, but no more arduous than planning any other restructuring of your garden. Good luck with your work! You can be sure of many enjoyable hours and years from your water garden.