Here are the things you need to check on each tab.
Transparency – Make sure that the radio button next to Existing image or layer transparency is selected.
Partial Transparency – On this tab, you may want to make several changes or none at all. Use the preview window to see how the default options appear on the After image. If they look fine to you (and in most cases, they probably will), go on to the next tab. If not, experiment with some of the settings until you're happy with the look of the After image in the preview window.
Colors – You can usually accept the default settings on this tab as well, but you may want to try selecting each of the different methods of color selection to see how the preview image looks with those settings. For instance, if you choose the web-safe color selection method, it may cause your photo to have a slight cartoonish look.
Format – On this tab, make sure that Version 89a is selected.
Download Times – This tab is for information purposes only. It gives estimates on how long it will take the final GIF image to load on a web page, depending on the connection of the person who is accessing the page.