My song, I bid that you express your sentiments with courtesy, for you must go among a haughty people whose wills are still so full of that ancient, most vicious of all habits, always truth's enemy.
Petrarch ends his canzoni with a congedo ("farewell") or envoi, in which he addresses his little song and gives it final instructions.
In this case, he's telling his song to behave itself in a "courtly" or sophisticated way when it reaches the intended audience, so that they will get the full meaning of his message.
He takes one last stab at the nobility by mentioning "ancient" and vicious habits—he means in-fighting among princely families—that keep them from seeing how their actions affect the country.
But you must try your fortune among the valiant few who love the good; tell them: "Who will protect me? I go my way beseeching: Peace, peace, peace."
This is an interesting route for Petrarch to take. Basically, he's telling his poem to act like a damsel in distress for those who would be likely to listen (i.e., the good guys). Apparently, everybody knows that knights in shining armor love to protect a lady.
That's especially the case when said lady has a message of peace. This is Petrarch's last bit of cleverness in the poem. By emphasizing that his message is one of peace, he situates the tongue-lashing delivered in the poem as necessary to a better world. Way to save your neck, P.