Creating custom brushes in Adobe Illustrator is easier than you might think.
Open a new document in Illustrator.
Draw a circle on the canvas.
Choose the direct selection tool and select the point on the far right of the circle object.
Drag the point to the right side of the canvas.
Use the drag handles on each of the four points of the circle to customize the shape of the teardrop.
Choose the selection tool to select the whole object.
Open the Brushes Palette and click on New Brush.
Choose Art Brush.
I used the settings shown in the image below. For brush scale options you can alternately choose Scale Proportionately. Click Ok.
You should now see your new brush in the brushes palette.
Use Your New Brush
Delete the brush object from your canvas.
Choose the Paint Brush Tool and select your newly created brush.
Draw on the canvas.
Calligraphic Style Brush
Follow the steps above for a teardrop brush but start with a shape similar to below.
Test your brush.