In this tutorial we will be working with Illustrator CS3. This is a very simple process with endless probabilities. This process will get you started in the right direction, and you can make it your own from there.
Need to know: There are four different types of brushes that you can make in Illustrator (Calligraphic, Art, Scatter, and Pattern). All have different setting and behave differently. We will be making an Art Brush.
Need to Know: There are four types of brushes that you can make in Illustrator (Calligraphic, Art, Scatter, and Pattern)
Getting Started:
1. Open a new document. Name it as you please
2. Create a shape (any shape at all). You can make a brush from ANY shape or group of shapes. It would be best to make something simple. After trying this a few times you will see how the shapes effect your brush.
Here is what I did:

ANY shape you want
Creating the Brush:
3. Open your Brush Palette
4. Select your shape or shapes that you want to make into a Brush.
5. In your Brush Palette, click the tab in the upper right, and select “New Brush”

6. Select “Art Brush” and click OK

Brush Options:
7. Under the Costomive box select Method: Tint. This will alow you to change the stroke color of the brush, other wise your brush will always be the color it was created in. You can play with the other settings on your own to customize your brush.
8. Choose your settings and click OK.

Your should see your brush in the Brush Palette
Using Your Brush:
9. Create a shape. I used a square.
10. Select the strock.
11. Select your Custom Brush from your Brush Palette.

12. With the stroke selected change the color by selecting from your Color Palette.
Below I duplicated the shape, changed the color, sclaed it, and rotated it.

13. Play with it until you get something you like.
14. Create more brushes and Enjoy!