outdoor sill???

i have a garden window on my 2nd floor. it has a crack in the outside pane of glass and i want to replace it with a double hung window. now i thought it would be easiest to buy a replacement window so i dont have to remove siding and lug a window up a ladder to install. I never installed a replacement window before and everywhere i read it says something about an outdoor sill. right now the window has no sill. its a flat 2x4 I already picked up the window cause i thought i could just take out the old one and slide the new one in and screw it and caulk the outside. but now im confused on the whole sill situation any help would be great been putting off this project for awhile