Hello Donna. There are many, many Canon lenses for the XTI camera. You can use either the "EF" or "EF-S" style lenses. Keep in mind, the EF-S lenses are for Canon D-SLR camera bodies that have a "crop" factor of 1.6. Your XTI has the 1.6 crop factor. The EF lenses will work on a crop body or a full frame style body. The 'image stabilizer' is a feature on some Canon lenses that will help 'stabilize' the image. The logic is that when you hold the camera, you have some level of "shake" in your hands and the stabilizer with eliminate that shake making your photo sharper. (It doesn't correct things if your subject is moving though. Only faster shutter speed or tracking will help that.) One final thing, the "L" lenses for Canon are their "pro" level lenses. That basically means they have a higher quality build and glass in them, but you pay a lot more for them. There are a lot of good non L lenses though.
Since you're looking for landscape and people photos, you probably don't need a lot of "reach" in the lens, but only you can answer if how long of a lens you're looking for. The first question is did your XTI come with a lens? If so, which lens did it come with? If it was purchased as the body only, then you might consider the new Canon EF-S 18-55mm f/3.5-5.6 IS lens. It is the new 'kit' style lens for folks on a budget. But by all accounts it's actually a very good lens for the price and it does have image stabilization (IS) in it. You can read about it here:
http://www.photozone.de/Reviews/Canon%2 ... rt--review
If you're looking for a lower light lens with a F2.8 aperture, then that changes things a bit. There's a few options, including higher priced Canon lenses or 3rd party lenses. I'm thinking the Canon 18-55 f3.5/5.6 IS lens should do fine outdoors and with people indoors using a flash. And with the image stabilization, if the people indoors aren't running around, it will probably do fine without the flash depending on how much light is available. I do swear by an external flash though as it seems to make a big difference. (For me anyway.) But the built in flash will get you by. I use the Canon 17-40 f/4L lens for indoor photos with a 430EX flash. I use a home made bounce card
on the flash and put the camera in manual mode and set an aperture of about f5.6 with a shutter speed of around 1/125s and get great indoor photos of my kids.
A lot of folks like Tamron, Sigma, and Tokina lenses too.
I'd forget listening to the sales clerks unless you know them personally and instead post questions here and you'll get a lot better advice from these folks. You can see reviews of many lenses at the photozone sight (and others). The main photozone page is: http://www.photozone.de/all-tests
Some of the info can be a bit technical, but most is easily understood. There's also end user reviews at Fred Miranda's site of: http://www.fredmiranda.com/reviews/