saving

i am new to mac, i am trying to save new created doc into my external passport. how do i do this and please explain step by step. when i get to where i want it i cant create a new folder the selection in file drop down is always grey i am unable to click that. any advice or explaination would be very help full. i am a phtographer and am always trasprting images from my camera to the external hard drive and a create new folders for each session to keep them organized. how do i do this on the macbook pro??? im so lost