Just got Rebel T4i... now what lens? software? how to shoot product photography?

[9VIII]

In my own (casual) testing the 18-55mm f/3.5-5.6 IS II is very decent in terms of sharpness (some reviews agree some not so much), CA isn't "as" controlled as on better lenses but it's still not terribly noticeable except in extreme circumstances (reflections of light sources on shiny things).

You're going to want a tripod for sure, I guarantee your pictures will be better, and unless the subject is moving that applies to photography in general (the reason you don't see people lugging them around all the time is that people get sick of lugging them around all the time). Get a remote shutter release as well (I use this one, it's cheap and it works).
A spare battery might also be a good idea, when shooting on a tripod I tend to use live view the most, and with live view on (which should be amazing on the T4i) I usually swap the battery after a few hours, which means that as long as you're not out in the woods you could theoretically shoot forever by just charging one battery while using the second (I've never actually had to swap batteries more than once in an afternoon).
For most "stuff" shots I try to find a well lit room and use a long exposure (usually around one second), I've never liked the look of flash. From what I've read most people use a remote flash off to the side with a diffuser if they do use one.
Oddly enough finding a room with the right light and enough space for your background setup can sometimes be the hardest part.

Just to help get to know your camera better, I suggest watching these videos, they cleared up a few odd points about the camera for me.
http://www.youtube.com/watch?v=iAx86nblZ2g

The best suggestion I can think of right now as far as camera handling goes is to become proficient at manually setting the shutter speed, aperture, and ISO. Eventually you'll probably just stick it in one of the automated settings, but having a good grasp on how those three settings affect the picture will make sure you know what to do if the camera isn't getting the shot right.
Oh, and having a grey card can be useful, sometimes the camera will display the colours all funny (I'm almost always on auto white balance), and just flashing a piece of white or grey paper in front of the camera can get it to set the colours more correctly.