I completely suck at DotA i am somewhat familiar with Nevermore. I can never kill any enemy heros even on easy. What strategies can i use to defeat enemy heros?

I do the basic get last hit to get cash and thats pretty much all i know. I buy the trent boots which is set to agility. The first two things i buy is a courier and the reg with +5 hp regen. I then get that sword with critical hits and upgrade it after getting demons edge.

I still suck and cant kill enemy heros even tho i have +130 damage (or more). What can i do?