help with creating transparent modles

im trying to create plants. i have some premade plants and tried to use them as an example and try to duplicate the effect of transparency between leaves and such. but with no sucess i read in the manual and searched for it. i figured out that i needed to somehow create an alpha channel in the paint program and import the image as a skin for transparency. i use microsoft paint and paint.net is there any way to do this with these programs and how? if not then is there a free program i can download that will do it? i dont want to pay $500 for photoshop.
or is there some other technique to achieve what im trying to do? any help would be appriciated.

thanks in advanced

try GIMP.
if you familar with Photoshop but want freeware then you can try GIMPshop.

thats a good idea ill give it a shot thanks