Illustrator - Create a Clipping Mask

A Clipping Mask allows you to fill a shape with a texture or image.

For instance I may want to fill a leaf shape with this image:

Open a New Document.

Select the Pen Tool (P)

Draw a leaf shape.

Now place the picture of leaves. File > Place

Go to Object > Arrange > Send to back and place it behind the leaf drawing.

Hold the Shift key and select the photo and the drawing.

Go to Object > Clipping Mask > Make