The latest versions of photoshop have useful tools for making gifs.
First off you can open a video file in photoshop, or you can open an animated GIF using the Open As dialog (open as a quicktime video), or you can select a sequence of images (file0001.png, file0002.png, file0003.png) and so on.
Once you have opened what you want to create a GIF out of, you can open the animation timeline (Window -> Animation) to adjust the timeline as necessary, set the beginning and end of the GIF, etc.
Then you can export the GIF using File->Save for web and devices, with a lot of advanced options for optimising playback and filesize.
That's about the bulk to it, there's more to learn but this should help you get started with taking simple video...