One of the most important things about growing a bonsai tree is choosing the right soil to use. There are a wide variety of soils available and choosing the best one is not always easy. You'll find many pre-mixed bags of soil that you can readily buy from a gardening shop but this can soon become fairly expensive. The main thing is to have the soil which allows for the plant to soak up all the nutrients it needs but isn't too dry as it starts to wither.
Many people make the decision to mix their own soil which works out to be cheaper in the long run. But this in turn can be quite complicated as it depends on the type of tree you are growing and how much water it would need. You can of course use simple trial and error to mix your own but this may mean you may lose some plants but on the plus side you will be gaining more knowledge and will be able to grow consistent bonsai trees in the future.
If you're not confident enough to try it on your own, then there are many resources you can use to help you. Your local gardening centre will be able to give you lots of great information and there are many books you can read to help also. In fact, a quick search on Amazon.com for the term "bonsai tree" results in dozens of books and in fact real miniature bonsai trees you can purchase. So there is no shortage of places to look if you need more information on keeping bonsai trees and more specific the type of bonsai soil to use.
It is recommend to use soil straight from the garden centre to get you started. This will give you the best chance of growing a natural tree if you haven't grown one before. There's no point spending time pruning a tree to look exactly as you would like only to have it die a few months later as the soil was incorrect. So get all the information you can then you can make the right decision to suit you.
Gardening Network: All about Bonsai trees