Dia dhuit!
I have got a best friend boy (his name is Ángel). I often say that it is my "anam cara", but I do not know if I should write "mo" or "m" in front of "anam".
For exemple: "Ángel, you are my "anam cara"? What is the translation on Irish?
Go raibh maith agat.

"Anam Cara" is not a term in Irish for soulmate. It should be "anamchara" if properly spelled, and even then it doesn't work for what you want most likely. There's a good post here explaining the nuances of soulmate, you should find it helpful.

http://www.bitesize.irish/blog/irish-endearments/

In connection with your post, "anam cara" in Gaeilge have any meaning? Syntactically, is not correct?
Go raibh maith agat.

"anamchara" (only correct spelling) is a "spiritual adviser" or "ghostly father"

A Angél, is tusa m’anamchara = Angél, you are my spiritual adviser.

Syntactically it's an absolute mess. Two Irish words jammed together using English syntax.

If you absolutely wanted to use "anam" and "cara" to say "soul friend," it would have to be "cara m'anama." But if you're trying to say "soulmate," there are many more naturally Irish ways to express that concept.

Redwolf

that's true most of the time, but in old words, sometimes one finds the elements put together in this order.
"Anam-chara" can be found in Dinneen's dictionary, it says:

"anam-chara [...] a soul-friend, a confessor, chaplain, spiritual adviser."

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I actually speculate in my book that the misspelling "Anam Cara" may have come from typesetters leaving the dot off the "C" in the title of O'Donohue's book "Anam Cara" (the original had a dot over the C, but subsequent editions and writings about it do not. Still wasn't hyphenated, though.

Redwolf