As usual, constantly build drones and send one out to scout if you want.
After your 10th drone, you'll want to make an overlord and then start up an extrator and put a maximum of 3 drones into it.
On the 14th drone, you should create a spawning pool for zerglings and a queen.
After another drone, you can either start some zerglings to get in some quick combat, or move out for a fast expansion and build another hatchery.
At 18th drone, make another overlord and a queen for each hatchery. If you think the other player will attack at any time soon, you'll want to build atleast two spine crawlers to help defend.
After your 20th drone, you'll want to get another extactor and a roch warren
Now you should should have a solid economy and army. You can go many many many directions from here. A basic build would be roaches and hydralisks. Zerglings will always be good in almost every build you go though. Make sure to upgrade the movement speed of zerglings if you plan to use them.