User blog comment:Kharjou/ADC tips chosing first big item/@comment-6464158-20130408204132

I think the general idea with item building as a carry is to start with either a Doran's Blade or boots, and then to immediately start building for major damage items such as an Infinity Edge or Bloodthirster. Focusing on attack speed is generally a bad idea in the early game because you can't poke very strongly (with autos or abilities, most carry abilities scale with AD) and last hitting is more difficult with less attack damage.

As for specific item building, if you start with Doran's Blade, you get boots and maybe a second Doran's Blade on your first trip back, then build an Infinity Edge, since you already have a little HP sustainability. If you start with boots, you would probably rush a Bloodthirster or something. On a side note, if you plan on building a Statikk Shiv, getting an Avarice Blade early would also be useful for the bonus on-kill gold and GP10 it gives.