One way you can say it, although its not very polite would be: Je m'en fous - this would be a vulgar way if you were mad. The best one to use would be - Je m'en fiche

You can also simply say, "Peu importe", which is more like "Doesn't matter".

"Je m'en fiche" is the best way to say it, you can also say "ça m'est égal"

You can also say "J'en ai rien à faire" or more vulgar would be "J'en ai rien à foutre".

In a formal way, use <<Ça m'est égal>> and it means "It is all the same to me", a very polite way of saying. In an informal way, use <<Je m'en fiche>> and it means "I don't give a damn" If you want to say "It doesn't matter", you would better off with <<C’est égal>>.