My starting point is usually like Eric's, f8 and 1/125. I use the meter to determine if I'm getting the amount of light needed for a proper exposure. If you use existing light f8 probably wouldn't give you a lot of light, so open the lens as needed (smaller f stop number=larger aperture/lens opening). I don't like to go slower than 1/125 shutter speed unless I'm in low light and then I don't usually set it slower than 1/60 handheld (and make sure I'm braced and have the camera steady).
Indoors I usually start at 400 ISO (I use manual settings) and raise it as needed. If you try setting the plant outside I usually start at 100/200 ISO in daylight.
You could try sitting the cactus where some window light will hit it, or put it under/near a lamp (you could try a few shots, with existing light and some w/a flash; I don't use a flash often, I've done sports where I have to use the available light so I've learned how to do that). I've heard of something like Designer described, I know a photographer who uses a white business card stuck in front of the built in flash on camera but I haven't tried it.
Where you choose to place the plant, think about the background and how it all looks in your viewfinder. Or if you'll be taking pictures of it where it's sitting, think about your vantage point and which direction will give you a nice background and some decent light. The cactus should make for a nice holiday picture.
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