Identify a tree in the forest, collect a leaf or needle and answer the tree identification questions. After the question interview is complete, you should be able to identify a tree's name at least to the genus level. You can also select the species with a little research using the links. I also provide my free TreeBook iPhone app as a companion pocket guide.
One caveat here. The tree you are trying to identify should be a North American native tree as defined by Forest Types of the United States. This key is leaf-based so review Identifying a Dormant Tree if your tree is without leaves.