Question

how do you write an ode poem of minimun of 15 lines/

Answer 1

Before you try to write one, be sure you are clear on what an ode is.
http://www.answers.com/ode

Are you supposed to write in the style of a Greek/Latin ode, a Spanish ode, an English ode, or what? Be sure to read the definitions and examples in this article: http://en.wikipedia.org/wiki/Ode

Answer 2

Learn the form:
http://en.wikipedia.org/wiki/Ode

next, study a few odes. Schillers Ode to Joy in Beethoven's Ninth Symphony is a masterful achievement.

http://www.bartleby.com/101/536.html

http://www.bartleby.com/102/171.html

http://raptusassociation.org/ode1785.html