A bonsai tree is not, in many cases, simply a dwarf form of a tree. A bonsai has been dwarfed by training. Training involves pruning, wiring, watering, repotting, fertilizing, etc. As you progress in learning about your tree, you will gradually become more aware of all the aspects of training and their use. For now, trim away any branches and leaves that do not add to the tree you are trying to produce. In order to do this, you must have a good idea of what it is you are trying to do. Take a picture (or draw one) of your tree before you begin work and draw a second, this time of how you want the tree to look in the future. You may or may not keep to this design, but it is important nonetheless to have an idea of the direction you want to take.
