I think the canvas color defaults to white, although when you
create a new image, you have the option of setting it to
To set the canvas color to transparent, select
Modify > Canvas > Canvas color...
and click the radio button next to Transparent.
If you have an image (like a bitmap or something) that has
white pixels and you want those to export as transparent, that's
set when you export the image as a .gif. Depending on your version,
File > Export (I think it was) or File > Image Preview
On that dialog, select GIF as Format, then Index Transparency
and choose the color you want transparent. Note that you'll get
haloing if you have any anti-aliasing to the white background.