Well, keep in mind that I have 0 ladder experience in Starcraft 2, but some general tips are:
- Always make more Drones, but have one to scout out your opponent. If you see your opponent switching to an aggresion build, stop making Drones and start making military units.
- Always try to fast expand as Zerg. You usually need to be one base up on your opponent in order to cement an advantage.
- One of Zerg's greatest perks is the ability to tech switch rapidly, by stocking up on larva. If you see a strategy you can't immediately counter, get the building that lets you counter it, then mass up on said unit (Mutalisks against a ground-only attack heavy army, for example).
- Queens. Queens are essential to a Zerg's dominance. Get used to putting your Hatcheries and Queens in Control Groups. This will help you play faster and more efficiently. :)
Also, there are several hubs that talk about this topic, so feel free to search them out and read them. :)
Hope this helps!