HI, I am looking to translate the term " you are my everything" for a tattoo.
I have seen many different ways of translating this and looking for someone who may be able to confirm if this is correct or point me in the right direction if it isn't.

This is what I have right now "Mo chuid den tsaol"

Any help would be greatly appreciated!

I am also wondering if someone has the correct way to write this in old Irish lettering. I believe it's called orthography.

Yes, expanding on what you already have, a common expression would be:
Is tusa mo chuid den tsaol mór.
You're my part of the great big world.

You can change to an older font using the font choices above in this forum:
Is tusa mo ċuid den tsaol mór.

Note that, in older writings, the "h" in "chuid" would have been left out, with the "c" having a dot over it, as I've shown. I've forgotten how to do that here, but someone else can help. What I did was to copy the letter "ċ" in from a Word document.

In case you decide to capitalize words, not that "tsaol" would be written tSaol", starting with a small "t", because the word "world" in this case is "saol", and the "t" is there for grammatical/pronunciation reasons.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

In your version "you" is missing.
Caoimhíns version can be shortened using a different word order which is more proclaiming:

Mo chuid den tsaol mhór thú!

Mo ċuid den tsaol ṁór ṫu!

There are some minor differences between dialects:

Mo chuid den tsaol mhór thú! (Ulster)
Mo ċuid den tsaol ṁór ṫu!
Mo chuid den tsaol mór thú! (Munster)
Mo ċuid den tsaol mór ṫu!
Mo chuid den saol mór thú! (Connemara and Official Standard)
Mo ċuid den saol mór ṫu!

"M(h)ór" (great/big) is not really necessary and can be deleted.

In case you prefer the old (pre-reform) spelling, "saol" must be written "saoghal", saoġal
And, AFAIK, combined words like "den" (of the) were seperated by an apostrophe: de’n, de’n
Regarding grammar, Munster dialect forms were often preferred as a quasi Standard.

So we get:
Mo chuid de’n tsaoghal thú!

Mo ċuid de’n tsaoġal ṫu!

Thank you so much. Your help is greatly appreciated !