When to build for
1-When your team does not have a reliable carry
2-If you that you can pull it off faster then other carries
3-PUBS(you should do this in all pub games IMO)
Basically you choose your stat boost of choice, 3 branches if you will build up a magic wand. Gauntlets if you will get bracers or Urn of shadows. Then you get a farming item of choice. Quelling blade if you're good at last hitting, sobi mask if you want to harass or just bad at last hitting, because whats the point if you end up not getting any last hits :S. Finally, pick you healing item of choice based on you preference, and how much gold you have left.
What lane you choose will define how you will be playing, Balanar at first, but it will eventually lead to this same mid-late game(No, not which build to choose. Just how you play this build). So what lane? What really decides where you should lane, are the other heroes in the game, not YOU. If the other team has a dominant solo hero you can't handle, don't be stubborn and let it go. If your team has a better solo hero, you should lane on a side also. Always give and take with your team remember that.
If you're solo laneing, i prefer you get a sobi mask because you will be nukeing alot. It's still up to you though. Hopefully someone on your team buys a courier, if not life will suck. Basically you should keep void at level 1 untill you're level 5, when you will start to harras your enemy with void. At level 6 or level 7 you if you have boots, and its night try for the kill. This is how it goes, keep void at level 1, but save the skill points untill your level 5, level up void. You start harassing the hero with it. Wait untill its night time and then you should head to base, go to a side lane and start to gank them as much as possible. Or in other words start to roam.
Before you roam your gear should look like this:
If you're dual laneing, you should get a queling blade, becasue you will need more surviability, and you won't nuke as much. Focus mainly on last hits, don't try to harras or nuke the enemy just farm with void, it will be very hard to get a kill untill level 7 realy. level void once or twice and then just use it to farm. farm farm because you wana try to get your core as fast as possible so you can gain the most out of the first night. the moment you get your first kill, is when you should leave your lane.