Small things that make a difference.

These tips are also applicable to other races.

As a Zerg player you should always Inject larvae with your Queen!
The amount of Larvae you can have without injecting is 3, by injecting larvae you gain an additional 3.
The max amount of larvae you can have it 17.

Always spend your resources!
There's no point of keeping unspent resources when you can be making units, structures etc.
Unspent resources slow down your army size, upgrades and structures.
Example: Getting a spawning pool 100 minerals after you have 200 will slow down the rest of your build and production by x seconds.
This is vital when the opponent is rushing.

Always scout!
You should be scouting early on in the game to see what the enemy is planning on making/doing.
Beginners find themselves getting contained by cannon rush because they don't scout the protoss, spine crawler and bunker/barrack rush!
Scouting should be done through out the game, you should not only scout the main but potential expansions.

Will be updating more tomorrow since I'm pretty tired.