Unit One: the eyes

There is no one way to draw manga/ anime eyes. The one thing that is important is that the eyes look like an actual organ used to see, not just an ornament pasted on their face. It's a good idea to consider these guidelines when creating your characters' eyes:

Boys/ Men

Males generally have smaller, more narrow eyes. Keep in mind, though, that rules are made to be broken. Oftentimes, manga artists will give their boys larger eyes to give them a younger or more feminine touch, such as Yuki Sohma in Natsuki Takaya's Fruits Basket.

Girls/Women

The one rule that must ALWAYS be followed when drawing any feature on a female is: Women have more curves than men. This even holds true for eyes. More often than not, girls will be given bigger, rounder eyes. Many prefer to give them smaller, more cat-like eyes. Even if that's the approach you take, they will still curve considerably, especially towards the outside end to give them some sort of eyelashes.

Age:

Eyes are important to determine your character's age, and vice versa. Below is a common pattern used for this:

Adults- smaller eyes, higher up on the face
Preteens- slightly larger eyes that sit just a hair above the middle of the face.
Children- large eyes that are slightly below the middle of the face.

Your Homework:
Practice until you're comfortable with these concepts, then put up a picture of one set of male eyes and one set of female eyes (don't forget to label them). Don't attempt to draw any other facial features. Put a link in the comments, so I can look at them and critique them.
Don't forget to have fun ;)