This board is a great starting place. One of the great things about learning here is, when you run across something you don't understand, you can ask the one who wrote it to explain further.
A Google search can turn up a bunch of info, too. The keyword "dendrology" might find you some basic info on structure and terminology. Look for sites with the .gov or .edu suffix for the best info. Using the keywords "trees", "identification", and "(your State)" should get you some info about trees you are likely to see in your area.
Right here at The Forestryforum is the best and most fun place to learn, though.
