Zerg players need to understand that the strength of the Zerg isn't building high tech units like Protoss and Terran
. The Zerg race is very good at swarming over a map, expanding fast and often. Zerg players should always try to have one more base then their opponent. This strategy guide will give you the basics to early Zerg expansion.
The most important thing to understand about an early expansion is that when you invest in that early Hatchery, you aren't getting units or upgrades. Scouting is crucial here. Scout your opponent early, if it looks like an early rush, expanding is not a good idea. Instead try and get your Spawning Pool down and brace for the attack. However, if you notice your opponent isn't going to rush, you should try to expand at around 14-15 population before dropping your Spawning Pool.
While your Hatchery is being built, your Spawning Pool should finish. When it completes, start building Zerglings and get Metabolic boost. We want to try and harass our opponent and keep him busy so he doesn't fast tech to air or try to harass your expansion. However he still may make it to air so its important you upgrade your Hatchery to Lair as fast as possible.
By the time you hit Lair tech you should have 2 bases operating while your opponent only has one. At this point it would be a good idea to get Hydralisks for anti-air. Keep harassing him and scouting him by sacrificing an Overlord. Don't let him expand! Force him to leave his base only to attack out when he runs out of minerals. By the time he makes this move you should be able to crush him with your superior force.