How does a Poet determine what form (say, a sonnet) that he or she is going to use ?

The poet considers what he or she wants to say, and if any form suits it. Sometimes the poet is writing a response to another poet, or a parody of another poem, and thus imitates that form. Other times the thought the poet is expressing naturally lends itself to a particular form—such as the Petrarcan sonnet, with its customary shift in thought between the octet and the sestet, or the obsessive repetition in the villanelle or pantoum. Other times the poet chooses the form first, possibly for a technical challenge, and the thoughts and words fill the form.