1)
What will he tell, and when?
1.
Rashi: From then (when He will make the Resha'im drink from the cup) and onwards, I will tell His vengeance and Gevurah.
2.
Radak: I (Yisrael) will always tell vengeance against the enemies that I see.
3.
Malbim: Straightness says, I will constantly tell Gevuros Hashem.
2)
About what will he sing?
1.
Radak: That he did not abandon [Yisrael] in Galus, and took His vengeance.
2.
Malbim: Song is always over a cup (Brachos 35a) - "Kos Yeshu'os Esa uv'Shem Hashem Ekra" (116:13). Straightness says, I will sing over the cup to inform people of the honor of His kingship.
