Basic Strategies

At the start of everything, you will need to collect sun which are falling from the sky and use them to build up your army to kill those zombies. On the start of the second level, you will be rewarded with 'Sunflowers', which give you extra sun. My advice is to start with 3 sunflowers and plant more after that.

Now, a zombie approaches. What do you do? Use your additional sun to help you plant a peashooter, which will help you kill the zombie.  This image should help you:

This shows the process of planting a peashooter, with the aid of sunflowers.

****Extra****
During the night, plant sun-shrooms. They are cheaper than sunflowers and give out sun pretty quickly. I heard that sun-shrooms hate sun.

Braaaaaains...