A summary, of course, is just a summing up of what the poem says, written in your own words. I'll give you the basics of the content of Henry King's, "A Contemplation Upon Flowers," and let you fill in the details. That's your job--I'll just make sure you understand it.
Stanza one centers on the characteristics of flowers. They are gallant, humble, etc., and they return to the earth (figuratively, they die every winter--but this isn't revealed until later) after putting on a show.
Stanza two switches focus to the speaker: he would rather it be always spring, so he'd never have a winter (again, so he'd never die, but this doesn't become clear until later: winter is often used as a symbol of death). He wishes he could go to the earth (his grave), and look as cheerful, and smile, as the flowers do when they go to their earth.
In stanza three, the focus on the speaker in stanza two combines with the focus on the flowers in stanza one, as the speaker asks the flowers to teach him to not fear death; to teach him that his breath may sweeten and perfume his death, as the flowers' breath sweetens theirs.
In short, the speaker envys the flowers their ability to face death without the fact of death negatively influencing their existence.
The poem depicts the ways that flowers handle their existence, inspite of impending doom; the failure of the speaker to handle the same situation as well as the flowers do; and a wishing for the flowers to teach the speaker.