does bonsai just mean that it is not full grown?
By the strictest definition bonsai just means a potted plant or a plant in a tray although most mean a potted tree.
Aesthetically, a bonsai should give the impression of maturity. One of the ways this is achieved is by maintaining a reasonable ratio between the size of the trunk and the height of the tree. A general rule of thumb is 1/6. In other words, a one inch trunk for a six inch tall tree looks good.
it is little ... maybe 2 feet high -- skinny trunk.... .. I would like to make it a Bonsai -- I do not know how to start
The usual method for trees that backbud is to grow it out until the desired diameter is approached then cut it back to begin developing taper and movement. A series of these cycles is repeated creating a tree with interest.
How do you know when the trunk has reached the proper dimension? You first determine how tall of a tree you desire, this determines how you should proceed. Have a look at this article. Once you read it, read it again, it really is an excellent synopsis of the process.