It is quite easy to start a bonsai, most of the information you need is here on BonsaiPlanet.com.
In Japanese, bonsai can be literally translated as 'tray planting' but since originating in Asia, so many centuries ago - it has developed into a whole new form. To begin with, the tree and the pot form a single harmonious unit where the shape, texture and colour of one, compliments the other. Then the tree must be shaped. It is not enough just to plant a tree in a pot and allow nature to take its course - the result would look nothing like a tree and would look very short-lived. Every branch and twig of a bonsai is shaped or eliminated until the chosen image is achieved. From then on, the image is maintained and improved by a constant regime of pruning and trimming. 
It is the art of dwarfing trees or plants and developing them into an appealing shape by growing, pruning and training those in containers according to prescribed techniques. 
Overall, bonsai is a great interest, hobby or even profession to undertake. Although famous theologians have claimed that it is actually 90% art to a meagre 10% of horticulture, it has to be said that a successful bonsai is most definitely a horticultural masterpiece. 

Here’s an introduction to the bonsai art:

The first thing to do when you are starting, is that you find some low-priced plants (at your local nursery, garden store or even in your garden) that have relatively thick trunks and have good bonsai potential that you can turn into bonsai through training (pruning, wiring etc.). Don't worry if your first few plants don't turn out how you want them to - with experience you will learn what and what not to do and thus improve them and your technique over time. 
Although bonsai is a very delicate and precise hobby in many aspects, usually the plants are very forgiving - so don't be afraid to prune. 
You can train the plants that you collect from nurseries, home etc… into any style that you may see fit. A good way to determine the style you may want to use is by looking at the plant and considering what style it may look good as and what it looks like when it naturally grows in the wild. 
When you have decided on a style that you want to turn your plant into, you can start pruning it - being careful that you really consider which branches need to go and stay so the plant can keep in balance and be pleasant to the eye. Try to prune the plant into a tree like form - or a form that is commonly seen in nature - to keep with the principles of bonsai. 
You do not need to prune your bonsai every day as many people think. Two or three times a year is enough - usually at the start of spring, end of summer and sometimes during late autumn or winter. 
Next - if you think it is necessary, you can wire your tree. Make sure that the wire you have chosen (copper wire is best) is the right thickness for the branches and is able to be manipulated easily. Be careful when you are doing this that you do not break the branches. Take the branch you are wiring into both hands and carefully wrap the wire around it. 
After wiring and pruning, choose a suitable pot. The pot needs to complement the tree and not be very big - usually the depth of the pot should equal the thickness of the trunk of the plant - but this law doesn't always have to be obeyed. 
When you are repotting make sure that you have proper bonsai soil at hand as well as small pieces of mesh to cover the drainage holes of the pot. Trim up to 1/2 (or 2/3) of the existing roots off - leaving most of the thin white roots (known as feeder roots) and some of the old thick roots. Make sure that the plant can now comfortably fit in the pot. 
Once you have worked out the position of where the tree should sit in the pot, (usually off-centre except for cascade and semi-cascade styles), put the mesh over the drainage holes and add a layer of soil for the tree to rest on. Once you have done this - position the tree in the pot and add the rest of the soil - making sure that you tap the pot occasionally so that there will be no air bubbles. 
After the bonsai has been potted you can now add moss or other small plants around it to give the impression of a fully sized tree in nature. 
Then just water the tree in and leave in a sheltered position where it can rejuvenate and you would have just created your first bonsai! 
Make sure that you research the specific requirements of the tree you buy and that you remember to remove the wire before it cuts into the tree - this can cause an ugly scar.
Whatever else you need to know - you will be sure to find it here - at BonsaiPlanet.