Here is a display of all the lenses available for the Canon T3i (with suggested retail price).
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup
I suggest you take a look at what is available but don't get in too big of a hurry to buy.
The lens you have will do everything you will need for a few months.
By waiting, you will know more about what you really want/need.
..
I am the proud new owner of a Canon Rebel T3i (with the 18-55mm lens), and I am looking for some advice / suggestions about purchasing lenses. Particularly macro lenses.

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