First, save your file as a PSD. That will keep it editable.
Second, find out how big the file is. GIFs typically only work well when they're small in size and short in animation length. If you're trying to make something, for example, 1000x1000 pixels with 50 frames, that's too big.
When you go to Save for Web & Devices, at the top right is a drop down menu. It will say something like Jpeg (Medium) or PNG-24. Click on it and select GIF. The default settings should be fine for you, but if you want, there's options that will appear once you change the file format just below that drop down. On the right side is an area for changing the number of colors in the image (256 is the max). If you have a big file, reducing the number of colors used can make the file size smaller.
Just to the left of that are options for changing whether it's transparent or not and a few other things you don't need to worry about.
The box with all the colors is showing you each individual color used in the image. You can use that box to remove specific colors if you want to, but you probably don't need to. Below that it will show the dimensions of the GIF, and if you look at the bottom left of the window, it tells you the file size and how long it takes to load in a browser based on different internet connections, which you can change. Most GIFs are under 1MB and ideally less than 500k. The lower the better. There's also a small button next to that for previewing your GIF in a browser. If it looks huge or takes a long time to load the animation, you know you need to change it.
Those are pretty much all you need to look for. If your file is too big, make the dimensions smaller and/or shorten the animation. Do that and keep checking the Save for Web & Devices window to see what it says until you're satisfied.
A common problem in Adobe Photoshop and Image Ready is being unable to save a file in the GIF format: Save As...: If you are trying to use the "Save As" command but there is no GIF option in the format drop-menu, then you are using the wrong image mode. In the file menu go to Image > Mode and change the mode to RGB or Indexed Color. Now try saving again — you should see GIF amongst the options. Save For Web: In the Save For Web window, look at the options on the right hand side. Make sure the format drop-menu is set to GIF. If it's not, you will only be able to save the file in an RGB format such as JPG. Image Ready: If you are trying to use the "Save Optimised As" option in Image Ready you may find a similar problem with a similar solution. Open your "Optimize" palette (Window > Optimize) and make sure it is set to GIF. once try these it may help....