Camera Shake
The rule (reciprocity rule) to avoid is straightforward but not always possible to follow specially indoors.
The focal length of the lens should roughly be your minimum shutter speed to avoid camera shake. So, if you are using a 50mm lens, the closest full shutter speed is 1/60th of a second.
However, in real life specially while using zoom lenses this will be very tricky to follow as you would not even know what your focal length is. For example, the kit lens shipped with most DSLR is somewhere around 18-55 mm. Now, this means if you are shooting wide angle at 18mm then you can get away with 1/30 or less. However, if you are zoomed in at 55 then you need over 1/60. So, in some sense you need to be at least 1/30 and 1/60 or 30 and 60 (according to what your camera will show you) to take shake free photos.
Now add ISO to this. These ratings are for ISO 100. So, if you shoot ISO 200 then you can shoot around 1/15 and 1/30 and greater on the same lens without getting a shake. And if you shoot ISO 400 then you can shoot 1/8 - 1/15 and higher and so on.
However, it will be pretty hard for you to shoot anything below 1/15th of a second and not shake your camera. So beyond that the ISO isn't really going to help.
Also, reciprocity law only holds between shutter speeds between 1 second and 1/1000th of a second.
Also, add IS (Image Stabilization) to this and it gets trickier. Generally, IS on the lens is going to give you 2 "stops" worth of light. Meaning to say that if there is enough light to shoot only up to 1/60th of a second then with an IS will probably help you to shoot at 1/15th. A very expensive IS might let you shoot about 3 stops lower.
If all that is too much then try shooting at 1/30 of a second or faster. One easy way to achieve that is to use shutter priority (Tv programmed mode.) Set the shutter speed to 1/30 or 1/60 and then let the camera set the aperture for you!
Use a tripod!
The focal length of the lens should roughly be your minimum shutter speed to avoid camera shake. So, if you are using a 50mm lens, the closest full shutter speed is 1/60th of a second.
However, in real life specially while using zoom lenses this will be very tricky to follow as you would not even know what your focal length is. For example, the kit lens shipped with most DSLR is somewhere around 18-55 mm. Now, this means if you are shooting wide angle at 18mm then you can get away with 1/30 or less. However, if you are zoomed in at 55 then you need over 1/60. So, in some sense you need to be at least 1/30 and 1/60 or 30 and 60 (according to what your camera will show you) to take shake free photos.
Now add ISO to this. These ratings are for ISO 100. So, if you shoot ISO 200 then you can shoot around 1/15 and 1/30 and greater on the same lens without getting a shake. And if you shoot ISO 400 then you can shoot 1/8 - 1/15 and higher and so on.
However, it will be pretty hard for you to shoot anything below 1/15th of a second and not shake your camera. So beyond that the ISO isn't really going to help.
Also, reciprocity law only holds between shutter speeds between 1 second and 1/1000th of a second.
Also, add IS (Image Stabilization) to this and it gets trickier. Generally, IS on the lens is going to give you 2 "stops" worth of light. Meaning to say that if there is enough light to shoot only up to 1/60th of a second then with an IS will probably help you to shoot at 1/15th. A very expensive IS might let you shoot about 3 stops lower.
If all that is too much then try shooting at 1/30 of a second or faster. One easy way to achieve that is to use shutter priority (Tv programmed mode.) Set the shutter speed to 1/30 or 1/60 and then let the camera set the aperture for you!
Use a tripod!