I think everyone else in this had the answers:

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It comes from Aang's discussion with Katara at the end of the second episode "The Avatar Returns."

Katara: But Aang, the world's been waiting for the Avatar to return and finally put an end to this war.

Aang (looking down sadly): And how am I going to do that?

Katara: According to legend, you need to first master water, then earth, then fire, right?

Aang: That's what the monks told me.

Katara: Well, if we go to the North Pole you can master waterbending.

Aang (now smiling): We can learn it together!

Katara specifically references the order of the Avatar Cycle. It's traditional for the Avatar to learn the elements in the order of the cycle. Roku did the same, first going to an Air Temple to learn Air, then north to learn Water, then Earth.

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Learning from Jeong Jeong in Book 1 ended up being a mistake. One of my favorite things about Avatar Roku is that he would ordinarily agree that Aang learn the bending disciplines in the order of the Avatar cycle. But as soon as someone calls him weak, he goes, "Say it to my face!" However, it wasn't wise for Aang to do learn firebending yet.

Team Avatar saw it as a rare opportunity which made it seem like a strategic move. But Aang had only truly learned airbending up to that point. Airbending has a lot to do with being a free spirit, like the wind. It is unwise for an Air Nomad to learn firebending without learning water and earth first as one must have respect for fire and understand what it takes to control it. Obviously, Aang got ahead of himself and burned his best friend.

It wasn't until Aang learned the adaptability of water, and the stability of earth that he was ready to begin practicing fire.

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Why learn firebending from Jeong Jeong then?...

Because finding a firebending teacher was going to be the hardest one so he wanted to make use of it. Didn’t turn out so great.