The first player will do a 10-pool build, meaning that they will build drones to 10 supply, followed by a Spawning Pool, then an Overlord, then Drones to keep larvae from being wasted (you want to make a drone or two before your pool finishes), and then 3 Zerglings as soon as your Pool finishes.
Meanwhile, your ally will fast-expand with a 14 hatch build and scout on 10. If he scouts an early attack, such as a double-10-pool, two gate zealot, 2-rax, he will 14-pool instead, and help you with the defense and counterattack. However, it is still likely that even if the map is large, your early zerglings will mess with at least one opponent's build, and if you don't throw away the Zerglings too quickly, your ally will have time to get his pool up even if he's doing a 14-hatch.
Things your Zerglings SHOULD be doing:
- Destroying Pylons that power key structures
- Harassing / destroying SCVs building structures
- Kiting Zealots
- Killing stray workers
- Making sure your opponents cannot easily combine their early forces (if bases spread apart)
- Getting pinned in the worker line (his workers will kill your Zerglings and you'll lose more zerglings than you'll kill in workers.
- Trying to kill more than 1 Zealot or more than 2 Marines.