The poem starts with a prayer by the narrator to the Virgin Mary, to help them be able to tell the story well. After that, they start the tale.
A long time ago, there lived a great monarch. His fair wife was called Donya Valeriana, and his name was Don Fernando. He had grown up in the Kingdom of Berbania, where he now ruled.
The king has three sons: Don Pedro, Don Diego, and Don Juan. The narrator elaborates on Don Juan a bit more, saying that he's pretty much like the sun rising over the land of Berbania and he's showered with much more affection than the other two. Needless to say, he's Don Fernando's favorite.
After the sons have received their basic education, they were asked by their father if they will choose to join the priesthood or train to become future rulers. The three all chose the latter, and so they were taught swordsmanship. And they did well.
Personally, I don't really subscribe to the Roman Catholic practice of praying to anyone other than God (I mean praying to Mary or the saints, which would make it idolatry if you think about it), although it is part of the majority culture here in the Philippines. But yeah, considering that this is gonna be a long story, I understand how it would be necessary to ask for some divine blessing.
Other than that... Reading this a little more intently makes it clear to me why the next scene happens the way it is.
And knowing a bit of what's ahead, I kind of have an idea on motive regarding a certain something that's going to happen.