|A lot of Canons come with what is called a "kit" lens. Usually in the 18-55mm. This a fairly good lens to get you started.|
This is actually such a tough question that many people won't touch it. There are different kinds of lenses for different uses. The 18-55 will give you a good range because the T1i is a what they call a cropped sensor camera. So your 18-55mm lens will act like a 28-88mm lens.
This is not an issue for most folks and as you become more experienced ( and richer :)) you can investigate full sensor cameras which are more expensive.
If you find you have to play and are on a limited budget, aren't we all, you can try a Sigma lens which I find are usually quite good and cost less. Most pros use the lens made by the maker of their cameras. Good glass, ( a lens) can cost more than the body but lenses can move from one body to another and are not for the most part affected by the features on the cameras themselves.
One tip, most lenses have a "sweet spot." I shoot all Canon lenses now and have found that F8 gives the best overall performance for sharpness and usable depth of field.
In the old days of film, the greater the depth of field the better the sharpness throughout the picture. Not so with digital cameras. Under certain circumstances, you may need greater or lesser depth of field, but because of the way the sensor, rather than the film, works these days, a mid range F stop usually provides better results.
And of course, just to keep things interesting, a lot of this flies out the window when you're doing macro photography.
So, I haven't really answered your question have I? Well, I can't really. It will depend on you and what you want to shoot. The 18-55 will get you started and should prove to be just fine. As you shoot, look at your shots on your computer. If you are not getting what you want, and it's not because you're not paying attention, then it will be to the boards to seek out the type of lens you want and need.
A general rule of thumb is the more light the lens gathers, say 1.8 as opposed to 4.5, the more versatile it will be. And the more expensive. Really expensive. You'll have to find what you can work with and what your budget will allow and work from there.
Remember, Ansel Adams shot all those great landscape shots with a camera that could almost be outdone a really good cell phone camera now adays.
Shoot and enjoy. And read, read, read the manual. For other than just snap shots, it's critical.
Human beings, vegetables, or cosmic dust, we all dance to a mysterious tune, intoned in the distance by an invisible player.