Nevermore is actually one of the more powerful heroes there are a couple things you should know about him:
Staying alive: Nevermore's damage is largely proportional to how many last hits you got since you last died. So stay alive! You're going to need Lother (which turns your invisible) or Kelen's (which teleports you) to do this, so after boots (str or phase) this is your first pick up.
Combo: because you have 3 shadowrase, you need to cast it more than once to kill. Figure out the middle range usage (hint its the same range as your basic attack) and then follow up with short (if he comes after you) or long (if he runs away). You can also combo it to get more kills.
Requim: This skill trades on being right next to your target, so combo it with Lother or Kelen's for maximum effectiveness.
- Necromasty (to start grouping up your last hits)
- Shadowraze (lvl 1 isn't great, so hold off on it
- Shadowraze (Your combo is starting to get good)
- Shadowraze (by now your combo is leathal)
10+ you want to max out Requim then Presence of the Dark lord
Damage: Most of your early game kills revolve around Shadowraze, but once you get to mid game you're going to want to pick up some dps. Demons Edge is very nice, as is Manta Style (with bonus to survive). Stygian is going to be essential in late game as it stacks with your Presence. Nevermore doesn't trade on his life steal, but Mask of Madness' on use ability is nasty. I usually go for Demon's Edge -> Stygian -> Buriza. If you're still looking for items after that Skadi or Satanic are both good, but you should be in end game by then.
Ok, it sounds like you have a larger problem than working well with Nevermore, that extends to DotA in general.
A carry is a late game hero who can push a lane and take out 3-4 opposing heroes with little support. A Hard carry is a character who needs to farm up all game specifically for late game. A Soft carry is a player who doesn't have a ton of late game abilities but can become very powerful late game due to ganking.
Ganking is a term for going into an ally's lane and helping to kill their enemy. This technique usually relies on surprise and coordination. Ganking is useful for all heroes as nearby heroes split the xp of a kill and the last hit gains significant gold. Agility based heroes usually operate as glass cannons early on, so ganking is a way to gain experience and gold with minimal risk (the risk usually being getting ganked).
Nevermore's ganking technique usually relies on casting shadowraze back to back. Ideally you can get an ally to stun the target and there fore get off all 3 shadowraze. Once you hit level 6, if you have full Necromasty use your lothar's to get right on top of an opponent and Ult. Follow this up immediately with shadowraze (remember to attack first so you're pointed in the right direction, but don't wait for the attack to go off). The target should be low and try to run. Just dps him down till he's in range of your next shadowraze and enjoy the kill.
Farming is another technique to get to late game with an agility hero. Nevermore is especially good at farming in his own lane (known as laning) because of shadowraze. Adjust your range to be ideal for mid-range shadowraze (you can do this by attacking). Wait till the last one of your melee creeps is about to die then cast shadowraze. This should kill some of them and injure the rest. When they come into melee range your second shadowraze will finish them off. This is excellent farming technique, but very mana intensive so stock up on consumables.