Apple -> System Preferences -> international -> Input Menu
Check the box next to 'Character Palette'
A flag will show up in your menu bar, click and and ask it to show the 'Character Palette'
You should be able to do basic work with Greek symbols, but you might find a real Equation Editor of use. Microsoft bundles one with their Office Suite.
Talk to one of your Math/Physics teachers at your school and see if you can get MathType for free. If not, iWork '09 comes with it built into Pages. Great program.
To write chemical equations in every application except Microsoft Word, all you need to write chemical equations are the keyboard and the knowledge of the keystrokes. You simply have to know the key combinations for the special characters that you want to use. To become familiar with them, enable the Keyboard Viewer in the International preferences pane. Microsoft Word grabs control of your special characters including the Symbol font and requires you to use Insert > Symbol... to insert them from its Symbol palette. To use an equation editor for chemical equations in high school chemistry is akin hiring a teamster to push your grocery cart in the supermarket.