The answer to this in early game is generally the standard 3-4 queen opening, a large number of Zerglings, and maybe a few Banelings or Roaches, if you'd like. Banelings and Roaches are usually not necessary with good Zergling and Overlord control.
When scouting, you're looking for an early gas- one started before the natural CC. If you see this, you can generally assume there will be pressure on your expansion of some sort, whether it's Reapers, Hellions, Banshees, Bio, or drop play. Focus on your creep spread and injects early on and by the time this attack hits you shouldn't have too much trouble fending it off just with basic units. Use your Queens to target down Medivacs and Banshees, and just chase Medivacs around with around 12-15 Zerglings each. He won't be able to unload and do damage if you're right on top of him. You're just stalling here, because you're a base up and gaining economically every minute he doesn't hurt you. Good Overlord positioning and map awareness can make it much easier to notice the incoming fliers preemptively- before they have a chance to start dealing damage. If you're finding that the drops are already fully unloaded before you can react to them, that's definitely what you need to focus on improving.
The only need you should have for static defense this early in the game is a Spine Crawler or two at your natural choke if you haven't been able to figure out what kind of attack is coming or for Hellion run-bys, or Spore Crawlers for cloaked Banshees. Good Zergling scouting just outside of your opponents natural should give you ample time to set up defenses if you see an incoming attack that warrants them.
