Repenting from shirk, and getting back on the right path

As salaam alaykum everyone, my name is Amir. I'm new to this forum but hopefully I can look forward to a good future here. This will be a long post, but please do try and read it all, this is really important to me, and help would be extremely appreciated.

Anyways, onto the matter I came here for. For as long as I can remember, I have disobeyed Allah (SWT) and all his Messengers (peace be upon all of them). Throughout the short 14 years of life I have had, I have committed many sins, some I am even ashamed to think about. Let me just show you a quick list:

-Eating/Drinking non-halal things (such as alcohol, bacon, pork)
-Using Allah's (SWT) name in a vain and rude way.
-NEVER once have I even TRIED to fast for the whole of a Ramadan.
-NEVER have I done Salah to Allah.
-Masturbated (very embarrassing)
-Shown disrespect to my family

Those are the bigger ones. But, the biggest one of all, was when I (like a fool), converted to Christianity because of a few stupid reasons I won't even bother boring you with. This lasted for a short month, after which, I tried coming back to Allah again, this also lasted for a short while. Sometime after this, you could say that I committed shirk again by going 'back' to Christianity. So you could say I have committed Shirk at least twice.

Here I am now, looking if Allah (SWT) will forgive me for all my betrayal, plus other sins.

So here is where you guys come in. I want to know what I should do from here so I can get back into Allah's good books. :)

Here is what I think I should do from here if I want to get to Jannah:

1. I say the Shahadah once again, with the utmost honesty and dedication to stick to it. Then, I beg Allah to forgive me for everything.
2. I stop all my sinful ways and habits.
3. I start giving Salah.
4. I apologize to all the family I have disrespected.
5. I do my best to fast in the upcoming Ramadans.

1) If I do all that I mentioned above, and abstain from sinful habits, but increase my good deeds, will Allah in all His wisdom possibly grant me a good place in Jannah?

2) If Allah accepts my repentance and forgives me, will he 'forget' that I ever committed the horrible sin of Shirk (plus all the others) and not hold it against me on Judgement day? Or will the Shirk and all the other sins stay on my 'record', even after sincerely repenting?

That is the end of my extremely long and boring post, lol. Please could someone answer the 2 questions above? And could you be sure of your answers and not just use guess-work. If you will be using your own opinion, please say so, so I don't take it as absolute fact.

Peace be with you all.