i think if u were gonna go for carry,get melee first,easier to time last hit and dmg also better...if really not good with it,start with quelling blade.
other than that,practice is all u can do to improve...
i start up as a carry,gpm so-so when compared to other ppl's carry...then recently i kinda try other role such as ganker/support...
well...its hard to change the habit of last hitting creeps though xD

those 2 are nice heroes.
just practice last hit