The main function of a flower is attraction. The enticing form, color, smell and the promise of sweet nectar makes resistance difficult for any pollinator or person. A flower is the reproductive part or seed-bearing portion of a plant, consisting of stamens, pistils, ovary, petals and sepals.
Look at a flower initially as a simple shape and identify which simple shape is most like your flower subject. Tubular flowers are one of the simplest flower shapes because the three-dimensional form is very evident. Here are some flower shapes to give you an idea.
To understand a flower, take it apart in a methodical way. Making an herbarium page will help you learn plant anatomy and the page will serve as a reference tool that will exist long after your live specimen. Learn more about flowers and how to create an herbarium page in Understanding a Flower, Lesson 5 in The Practice of Botanical Drawing.