Hi Yeochinch, the first thing to do is practice holding the camera steadily in both hands (left hand supporting the lens) and pressing the shutter release button gently, breathing out as you take the shot - this can greatly reduce camera shake.
But as far as settings are concerned, you'll need a sufficiently quick shutter speed to eliminate camera shake. This varies depending on focal length, general conditions (is it cold and you're shivering!) and of course your own natural shake. As you take more photos and get to know your camera and lens, you'll figure out which shutter speeds are safe to handhold, which are on the cusp of shake, and which will definitely be too slow.
As a general rule of thumb, most people can handhold a shutter speed equivalent to 1 over the focal length. So if you're using a 55mm focal length, a shutter speed of 1/55 (1/60 is closest), would be best. Of course most DSLRs effectively multiply their focal lengths by 1.5 or 1.6 times, so 55mm on your 400D would actually be equivalent to 88mm, so you'd be looking for a shutter speed of 1/88 (approx 1/90 or 1/100) or quicker.
So if you're using a typical 18-55mm zoom lens on a DSLR, you should be looking at a shutter speed of 1/30 or quicker when zoomed-out to wide angle, or 1/90 or quicker when zoomed-in to telephoto.
Once you know what shutter speed you need, you can put your camera's command dial to Shutter Priority (S) and enter the desired speed. A shutter of 1/30 is represented by the number 30 on your camera, while 1/90 by 90. If you see inverted commas, like 2", this means 2 seconds.
In Shutter Priority, your camera will choose an aperture to match your shutter speed, but if there isn't enough light, you will need to increase the ISO sensitivity. Be warned, this will reduce the quality, so only increase it if you REALLY need to to avoid camera shake, and make sure you put it back to Auto or a lowish number afterwards.
If you're experiencing camera shake and want a quick fix though, just try increasing yuor ISO to, say, 400 or even 800 for that shot. as it'll allow your camera to automatically select a quicker shutter. Don't forget to put the ISO back afterwards though!
An anti-shake lens should allow you to handhold at around 'three stops' slower than normal. So if you really did need 1/100 for those telephoto shots, it should let you take them at 1/12 instead. Obviously this won't freeze any moving subject though.
But don't forget your technique, or something as simple as leaning against a wall or fence. All these things cna let you handhold more easily and better avoid camera shake.
Hope that helps!