Hi everyone! I wanted to get a tattoo in Irish, I know it is not that coherent to get a tattoo in Irish if the person that wears it is not able to speak the language. But I really want to get something that only few persons could understand. The sentence I want to get tattooed is: . If you could/want to instruct me in the irish grammar I would be so thankful, as I would like to know why do the words go in that particular places. Thank you so much!

I think, better English would be you "will" be my guiding light here, but wait for more input on that. I will assume it to be the case for the Irish translation.

Nuair is gá dom dul abhaile, beidh tusa i mo sholas stiúrtha

Wait for more input/confirmation on that suggestion.

As I've said more than once, in this construction the pronoun and the possessive adjective must be in the same person: mé/mise i mo, tú/tusa i do, sé/eisean ina etc., so:

beidh tusa i do sholas stiúrtha agam or beidh tusa mar sholais stiúrtha agam.

I don't see anything wrong with using the present tense (though I know what you mean, Gumbi), since it's referring to a general occurrence, so bíonn rather than beidh.

Gingerhead, you'll need further clarification, so get back to us.

(Got caught in mid-edit)

As Errigal said, beidh tusa i mo ...doesn't work.
It could only be used in the very literal sense: "You will be in my light".

I'd suggest:
Nuair is gá dom dul abhaile, is tusa mo lóchrann.
(lóchrann, lantern, torch, acc. to FGB and téarma.ie "guiding light" as well.

or (perhaps even better)

Nuair is gá dom dul abhaile, is tusa mo réalta eolais.
(réalta eolais, "star of knowledge", acc. to EID figuratively "guiding light" as well.)

grammar:
nuair = when, followed by a relative clause
is gá dom "is need for me", one of many ways to say "i must", "I need". "Is" is already a relative form, so no change: nuair is ...
dul abhaile = "to go home" (no need for "to" in irish)
is = "is" ("am", "are" as well); there's no future form for this little "verb" (rather a particle in Irish). But there's no need for future tense here, because "when" gives enough information to know that the sentence is referring to the future.
tusa = you, an emphatic form of tú
mo = my; a following noun must be lenited, so solas -> "mo sholas" = my light. But l and r cannot be lenited, so "mo lóchrann", "mo réalta".

Go raibh maith agaibh... Déanfaidh mé sár iarracht gan an botún sin a dhéanamh aríst./

@ OP, I second Labhrás suggestions.