Planting the right tree in the right spot will help provide the proper moisture for your tree in the long term, but for a newly planted tree just starting out, it is sometimes necessary and usually beneficial to provide additional water to establish your tree. The rule of thumb is to have about 1 inch of rain per week for your tree, or about a gallon and a half for each inch of trunk diameter every day. You should water daily for the first two weeks, with about 1.5 gallons per diameter inch, then water every other day for the next two months.
During the summer months, add about 10 gallons of water to a 1-inch caliper tree each week, in the absence of rain. After two months, you can water weekly until the tree is established.
Proper mulching is a great way to conserve soil moisture and facilitate moisture entry to the roots. Regularly check soil moisture by digging a shallow hole next to the tree to see that you are not overwatering the tree. Many trees have been "watered to death." While some water is good, in this case, more is not better.