Correct me if I'm wrong

Question

Welcome fellow puzzler, be thrilled with joy  
Letters and numbers pair up in today's alloy  
Go ahead and fear not - only enjoy  
You may need more than one try but that's okoy!

Let's begin with the very first of the set
Now from Fort Ian use all you can get

Go on with the first in your mind
Make sure to leave one more behind

Not only first but the last at rear
Add some bad luck to make it sincere

Think of my age quite literally and
Add once the number in your hand

You may ask yourself why? Well, it's true
Take poo to the poo to the poo, will you?

Next is the first one within a golden cup
So we try one on one but do not add up

Used for parking and the number one
Almost thirsty but with half of it gone

It may occur like one vacuous rubbish to you   
But maybe it's time to redo the maths of yours?  
Listen up now, you are the one I'm talking to  
Proceed until the code is correct, of course!  
(and then tell me the code and all that)

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Hint:

- In order to solve this riddle you don't have to perform any impressive stunts with real poo
- Hints can be found everywhere

Answer 1

Changing my answer once again.

I think the final answer is: 12,25,13,21,10,15,11 (see end of post). The final lines instruct us to "proceed until the code is correct (also mentioned in the title). So we take the code we got, ONEMORE, and write down how many letters we need to move forward to change it into CORRECT. This gives 12,25,13,21,10,15,11.

I think the partial answer is

ONE MORE

Let's begin with the very first of the set

First letter of alphabet is "A"

Now from Fort Ian use all you can get

Fort Ian is 14, A + 14 = O.

Go on with the first in your mind

First letter of mind is "M"

Make sure to leave one more behind

M + 1 = N

Not only first but the last at rear

First and last letter of rear is "R"

Add some bad luck to make it sincere

13 is unlucky. R + 13 = 31 mod 26 = E

Think of my age quite literally and

Age sounds like "H"

Add once the number in your hand

5 fingers in your hand. H + 5 = M

You may ask yourself why? Well, it's true

"Y"

Take poo to the poo to the poo, will you?

Poo is number 2, 2^2^2 = 16, Y + 16 = O

Next is the first one within a golden cup

Golden cup is grail, first letter is "G"

So we try one on one but do not add up

11 (Don't add up the ones, put them together), G + 11 mod 26 = R

Used for parking and the number one

P (P for parking, and pee for number one).

Almost thirsty but with half of it gone

30/2 = 15, P + 15 mod 26 = E

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Explanation

"ONE MORE" is the result of the letters and the numbers given by the riddle. "ONE MORE" wants you to apply the code again to the string we just got. So we use the code again to make our result "correct".

Summary

AMRHYGP + (14 1 13 5 16 11 15) = ONEMORE
ONEMORE + (14 1 13 5 16 11 15) = CORRECT

Final code

CORRECT = (3 15 18 18 5 3 20) You got the idea right so I accepted (although I actually wanted the above code from "correct").

Answer 2

The answer is:

L D K J F J M O (0) C M (39/13)

Let's begin with the very first of the set

L, 12

Now from Fort Ian use all you can get

Fort Ian is 8 character, 12 - 8 = 4

Go on with the first in your mind

M (mind), F (first), 13, 6. 13 + 6 = 19 - 8 = 11

Make sure to leave one more behind

11 - 1 = 10

Not only first but the last at rear

Rear = 4 character, 10 - 4 = 6.

Add some bad luck to make it sincere

luck = 4 char, 6 + 4 = 10

Think of my age quite literally and Add once the number in your hand

a=1,g=7 and e= 5. 7 + 1 + 5 = 8 + 5 = 13

You may ask yourself why? Well, it's true

In programming, true = 0.

Take poo to the poo to the poo, will you?

poo poo poo = 3 (You said poo 3 times)

Next is the first one within a golden cup

When You get win, You get a golden cup. So, Win -> W

So we try one on one but do not add up

13+3(poo poo poo)+23(W)=39

Used for parking and the number one

P, the one letter in parking.

Almost thirsty but with half of it gone

39/3=13, M.