How to know what settings to use?

[iolair]

If you're handholding, make sure you you don't exceed 1/lens-length s if you can help it. (For example, with a zoom lens at 300mm, don't hand hold shots slower than 1/300s).

I'm going to have to disagree with you. I've gotten great shots at 1/200 at 400mm.

The problem with a slower shutter speed at long focal lengths is that any shake is more noticable. You just have to make sure your hands are steady enough or shutter is fast enough to get a clear shot.

Good But it's a useful rule of thumb, and a first place to look if you get blurry shots at slow shutter speeds.

I remember someone on here a month or two back posted crystal clear shots from a 50mm lens at 1/20s. For most people, though, the 1/focal-length rule is very useful. Experimenting and finding out the limits of your own equipment and skills is even better.