"I don't care." Should be used carefully, as it could have the connotation of a negative attitude in some situations. It's not a phrase I use casually. I might say, "Anything is fine with me," instead.

Hello! It's the difference between 'no me importa' (I don't care) and 'no importa' (it doesn't matter). The second one has to be IT, not I. It's a third person expression. I hope that helps!

If someone asks you a question like, "what do you want to do tonight?" then you can say "I don't care" to mean you are fine going where.