As a general rule, you want to keep the ISO as low as you can, given the amount of light you have available, and the speed you want the shutter at. Higher ISO = more noise in the image, which depending how bad it gets, you'll either have to process out in some program after the fact, or just live with. I've not used a 1000D but going on my 450D and 550D experience, you won't want it higher than 400 for normal use, really.
As for the shutter/aperture, entirely depends how much light you have available and how fast the thing you're trying to shoot's moving. Experiment :)
If you're new to manual settings I'd stick to Av (aperture priorty) or Tv (shutter priorty), depending on the situation. If you want to control the depth of field (how blurry the background is) us AV with the smallest F# for more blur, larger for less. If you're taking picture of things moving fast then stick on Tv and increase the shutterspeed to your liking. The camera will balance the settings for you with these settings. You'll need to set the ISO though...but as a general rule, have it between 100-400, or just stick it on autoISO.
Keep an eye on what settings the camera chooses while you are in shutter and aperture priority modes for a while you'll get a feel for what to do yourself. Personally I don't go full manual unless I'm looking for a particular effect.
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As for the shutter/aperture, entirely depends how much light you have available and how fast the thing you're trying to shoot's moving. Experiment :)
(I have the 500D)
Keep an eye on what settings the camera chooses while you are in shutter and aperture priority modes for a while you'll get a feel for what to do yourself. Personally I don't go full manual unless I'm looking for a particular effect.