A Zerg Vs. Zerg mirror match is one of the hardest match-ups on Battlenet right now.
The Zerg race specializes in countering their opponents units, when two Zerg go against each other it can get very challenging countering someone who is countering you. However, if you keep scouting and focus on building the best units, you should be able to win. This Starcraft 2 strategy guide will teach you how to win a Zerg Vs. Zerg mirror match easily.
There is a few important things you want to keep and eye out for in a Zerg Vs. Zerg game. The first thing that is very common in Starcraft 2 is a Speedling rush. Make sure you scout your opponent very early so you know how early he is placing his Spawning Pool. If your opponent drops a Spawning Pool before you are at 12 population, he is planning on rushing you with Zerglings. To counter this you should get Speedlings of your own and 2-3 Spine Crawlers. Wait for you opponent to attack you, when he does, your Spine Crawlers will rip his Zerglings to pieces. You might be able to counter him after this and win. If you think you need to wait you should try and get some Roaches to secure the win.
Another less common strategy a lot of Zerg in Starcraft 2 try is the Roach-Hydralisk combo. The key to defeat this is build to Hydralisks of your own with some Banelings. Also try and get 4-5 Mutalisks to harass his Drones with and keep him distracted. Pick him apart with your Mutalisks until you have a bunch of Hydralisks and Banelings.
Avoid massing a lot of one particular unit because your opponent can easily counter one unit. Try and have a diverse set of 3-4 units to give yourself flexibility to change your build if you need to.
You can find many more Starcraft 2 strategies and reviews of some of the most popular Starcraft 2 guides at my website.
Best of luck to you. I look forward to seeing you on BattleNet!