How can I come up with a working thesis on the Dylan Thomas poem "Do Not Go Gentle Into That Good Night"?

You could certainly address the fact that the speaker uses myriad types of men to prove to his father that everyone, no matter how wise they are or how well they've lived, fights death in the end; he then uses this argument to claim that his father ought to fight too.  

He argues that, although they know they must and should die, wise men fight death because they feel they've not had enough impact on the world.  Good men fight death because they want more opportunity to do good works in the world.  Wild men who seemed to make the most out of every moment, regret, in the end, that they did not, and so they fight death as well.  Finally, serious men, nearing death, realize that they could have been happier during their lives, and so they rage against death.  Finally, then, he says to his father, "And you, my father, there on the sad height, / Curse, bless, me now with your fierce tears [...]."  He's speaking to his father, who is just about to die, and he begs him to fight fiercely, though it will be both a blessing and a curse.  A life extended seems like a blessing, but pain or disease extended seems like a curse; still, the narrator selfishly desires his father to fight death and has tried to convince him that this is what everyone -- including the best men -- do, and so he should do it too.