Two(2) iphones on my iphone? Help me!

I have two(2) iphones on my iphone. Let me explain. First off this is my first time having an iphone, so can you please be detailed as you possibly can, thanks! Its an Iphone 6, latest update iOS 9. My first iphone got broken, so i bought a new one. On my first one i backed up everything to my icloud account as well to itunes. As for my second one. When i got it i restored it from my icloud account. I seen it didnt give me everyrhing that was on my first one, i didnt even bother trying to fix it, i jst let it be. Before i updated to the iOS9 i wanted to back up my phone jst in case. As i was in the process of doing so, i seen two(2) iphones in my icloud account and one happend to be my old phone. When my phone restarted, i picked my old iphone account, only to save everything again and then i wanted to switch phones and erase my old one. I havent been able to switch back to my new phone. It's kind of frustrating, heres why. Like i said I let it go when i didnt get everything from my old phone to my new one, therefore i started buying a lot of music from itunes, started downloading a lot of apps, taking a lot of pictures/videos, etc. Here are my questions...

1) How can i make sure everything that is on my old iphone, transfers to my new one? If possible.

2) How do i transfer back to my new Iphone?

3) How do i delete my old Iphone from my icloud account?

I also would like to know....as of right now im using my old Iphone memory,data or whatever its called. I need to use my email. I went to the mail app(blue one) it gave me opptions the icloud, yahoo, gmail, etc. I tapped the icloud one and tried loging in. My phone said that, that email was already added to my Iphone, ok. Then why am I not recieving any emails? My question is...

1) How do i log my icloud email onto this old phone account? If its already saying its already being used.

Ok. Last problem/question. As for my music. So far I bought all my music from itunes. On my first phone. I bought my music, i backed up my phone to icloud. Then i had to update my phone to the iOS8. Once I did that, when i went to my music, I could not play any of it, unless i was using wi-fi OR I made it offline, but even then it had to re-download all my music. Here is my question...

1) Is that going to happen all the time, when i update my phone?

(It happend again when i updated to iOS9 and when i switched phones).

2) If its always going to happen...how do i make it stop? if possible

I also just want to double check im doing everything right. I see that i kind of am. Only cuz i see somethings havent erased from my old iphone, but then again somethings did get erased. Here is my question.

1) How do i backup/restore my Iphone the right way?

I want everything...if possible. My pictures, videos, apps, contacts, music and anything else that i need.

All advice is accepted. Thanks to whoever fully helps me on this as well to those giving me advice or feedback. I appreciate it.