I'm trying to be a better Zerg player. What are the pros doing for openings?
(this is specifically for the original StarCraft)
There are so many different build orders to use against each race it's ridiculous. Also consider that good players will be able to shift their build to better react to their opponents.
That being said, I think the most common one is some variation of a 9 pool build.
I should warn you that I am no SC expert player, or anything. I get most of my info from http://wiki.teamliquid.net/starcraft/Strategy_%28Zerg%29
Knowing build orders, knowing when to use a build order and knowing when to change your build order on the fly are all very complicated questions with very complicated answers. If I were you, though, I would start with the 9 pool. Learn it's advantages and weaknesses over other builds. Also you should think about what the build is capable of, what you are able to tech into from this build and etc.
Because of how different the races are, there really isn't a single opening you can use for every game. You can do quite well though if you pick one build order for each race matchup and get good at it.
Against Protoss, the standard opening is the 3 Base Spire into 5 Hatch Hydra build. It's an economic build that starts with a quick expansion, spawning pool, and a second expansion. After that you get a spire so that you can use scourge to defend against corsairs, add two more hatcheries, and start building up an army of hydralisks.
Against Terran, the two standard openings are 2 Hatch Muta and 3 Hatch Muta. The 2 hatch variant is more aggressive and requires good mutalisk control to do early damage, while the 3 hatch variant is more defensive and sets you up for a strong hive-tech game. Both are focused on pressuring with mutalisks while taking a third base and defending it with lurkers, and teching up to defilers.
Against Zerg there is a variety of viable openings, but the two most popular builds are the 9 pool and 12 pool. A 12 hatch opening is also viable but is more risky than it is in the other matchups. ZvZ matches typically involve using zerglings for pressure and defense while teching quickly to mutalisks.
I just registered my account so I can't post multiple links, but details for all of the builds I mentioned above can be found in the Zerg section of Liquipedia - look under the Specific Strategies section. While you're there, skim through the entire Zerg Strategy area. There's a lot of useful information.