ive got a noob question

' i just started to do my own pokemon rpg, of course with this starter kit. i have worked with the rpg maker yeaaaaars ago XD so thats why i think its a noob question.
when i try to create those trees, i know how to set the tops that i can run behind them, but how do i set them transparent?? i always have them with a white backround and that doesnt really looks good on a green field...