The Dark Bow is a Ranged weapon that fires arrows. However, you will not gain any ranged exp when using it, but instead will gain exp the way you would if attacking with a melee weapon such as an abyssal whip. It can be bought from the shops teleport. It is also a drop by most monsters. if you want to level up range there is a target in "train" which in in your emote tab (you dont need a bow for this). Repeat clicking the target till you get 99.