Select one basic idea from "Death Be not Proud" by John Done, and then find quotations in the poem, which prove that what you are saying about it is correct.

This assignment requires a student to select a significant idea from John Donne's Holy Sonnet "Death Be not Proud" and then use quotations to support some interpretation of that idea. It is part of a sequenced series of assignments; although the eventual paper will use three quotations, at this particular stage, the student simply needs to select the basic idea or thesis that will underlie the paper and locate one supporting quotation.
A good choice of central idea would be that Donne considers Death a "slave" to Fate. This terminology and Donne's account of various forms of death, would recall to educated readers of Donne's period the Boethian distinction between Providence (God's overarching plan) and Fortune or Fate (how humans see the events of the world). The important quotation for this is:

Thou art slave to Fate, Chance, kings, and desperate men,
And dost with poyson, warre, and sicknesse dwell

This suggests that Death lacks autonomy and thus should not be respected. In these lines, not only rulers but also "desperate men" (criminals, pirates, mercenaries) are shown as having control over the moments when people choose to die, and thus Death is subservient to their whims.