'-(으)니' in Seung Ki Lee's song '지금부터 사랑해'

Question today comes from Tan in Malaysia.

I'm now trying to understand the song "지금부터 사랑해" by 이승기,
and of course.. I'm facing some problems. :( Please help!!!

"지금 와서 보니 추억도 많더라"

- I'm eager to know how to use "보니", I know "보다" means "to see", but I wonder its usage right here means:

→"looking back"??? as there is an "오다" [back] before it... OR,

→ the "보니" is actually a grammar point which indicates "After doing..., I found out that...", or only the "-니" is having this indication, not the whole "보니"?

Many thanks!

Which Tone of Speech Should I Learn First?

Question today comes from Junil Jung in Chicago, Illinois in U.S.A.

I am studying to go to Yonsei Korean Language Program in summer. I took a look at their textbooks, and they were teaching -니다 form. I read your note and understand the difference between -니다 and 아/어요 form. My question is, which one should I have learned first? 

Tone of Speech 101

Question today comes from Johanne from Austria:

My teacher taught me -요 form. But when I talk to Korean people, they keep on correcting me and ask me to use -다/까 form. I am confused. What's the difference?