Depends on how keen you are on collecting several lenses and how much money you want to spend.
Cheepest and not the worst possibility would be to buy the EF-S 55-250mm IS (approx. $200) as a telephoto extention of your 18-55mm kit zoom, which is quite good in its range.
If you only want to use 1 lens, try the EF-S 18-135 IS for approx. $400. That would also replace your 18-55mm.
The quality of both these solutions is not professional but generally more than sufficient for general use. Low light suitability of both setups is not so great. However, to achieve this you would have to spend MUCH more money. Have a look at the various 70-200mm f/2.8 zoom lenses. I think they start off at about $900.
An excellent low light addition to the cheaper zoom lens setups is the medium telephoto prime lens EF-S 50mm f/1.8 II for about $120.
- -Better a small camera in the pocket than a big one on the shelf --