This is a sad fact of the activity, especially as you start out. You are dealing with living things, and you must be respectful of that. Commit yourself to understanding why every tree dies and what can be done to prevent it. Learn from your mistakes and do your best to prevent them in the future. Try to limit yourself to a few kinds of plants.
Every tree is different.
Likewise, every kind of tree is different as well. Maples have differnet requirements from Junipers, which have different requirements from Serrisas, and so on. Learn to care for a few different types of plants, and grown your collection from there. Find a club or organization in your area and join. This is, undoubtedly, the best way to learn bonsai.