Here are my questions.....If I went to my local home improvement store and bought a tree that is already somewhat grown, maybe something that is ten to fifteen inches high. Could I create a bonsai out of a tree that has already started to grow? If it is possible, what would be my first course of action? keep in mind, I am new to the bonsai world. Any step by step replies would be appreciated.
Thanks,
abran

Most of the time a 'home center' is the worst possible place to buy a tree/shrub of any kind. Try a 'real' nursery they will provide you with some information about the tree as to bonsai and some people there do care.

Bob

Sunset Publishing puts out an excellent text for beginning aspiring artists. The title is simply "Bonsai". Early editions of this book are poor, but the one by Susan Lang, published in '03, ISBN #0-376-03046-1 is very good and presents a good overview.

Al

Short answer to your question is, yes...but with some caveats.

First, as Bob said, home improvement stores won't have the best plant material.
And what they do have will most likely be stressed from bad soil and potting
conditions. So, if
you can, go to a nursery proper. However, even at a nursery, you might
encounter plants that are root-bound...if the roots haven't been tended
as the containers have been sized-up.

Secondly, as Al says, you need to understand plant physiology and soil
dynamics. Al's Threads in the Container Forum are invaluable resources.

Thirdly, most bonsai are grown outdoors.
Smaller containers are more affected by hot and cold extremes. So, where
do you live? What are your options in terms of plant hardiness?

Finally, be advised that High Summer (in north America) is not the time to be
disturbing roots; for instance, planting or re-potting trees, et cetera.
If you buy a plant now, it might be best to keep it in its pot, in a mulch-pile
perhaps, until next Spring.

Josh