Before you can properly care for a tree and/or identify why it is unhealthy you need to know what kind of tree you have. There are a variety of characteristics such as the leaf shape, leaf fall color, flowers, fruit, buds, bark, and tree shape that can be utilized to identify different tree species. This site will provide information on many different tree characteristics, and links that will actually help identify the trees in your yard.
Key to Trees of Iowa
Identification of Hardwood Trees in Iowa
Identification of Conifer Trees in Iowa
What Tree Is That? (Arbor Day Foundation)
Tree Identification Links