The canon 18-55 lens is usually enough. If you put it on manual focus and scroll right in you can get pretty good macro shots. I wouldn't buy a specific macro lens unless you wanted to use it for other things as well.
The thing to remember in macro mode is that the area of an object that is in sharp focus will be quite small, so you could, for instance, have the head in sharp focus the arms and legs may be blurred. You can prevent this to a certain extent with a very small aperture, but then you need a lot more light.
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An example of a wide aperture macro shot
And an example of a narrow aperture shot. If you click on the pictures the dakka gallery has all the camera settings I used at the bottom of the page