Transparent Background

Hello
I have a square picture. I have pasted a cut out of a leaf there. I want to save the picture so that the background is transparent. I have played with every transparent knob in the program and it still don't work??

basically, since the canvas is square, how can I make the image just the cut out of the leaf and not the "empty" fill around it?

Thanks
:confused:

i guess
this question is more asked then "what is the meaning of life"
there must be hundreds of these posts

lets see if i can remember what the last one said:

start as trans. bg
then start on above layer1
then save for web > matte = none
transparent gif

i may have jumbled that up a bit, but i think it's correct...