"lo porto nel mio cuore" or "la porto nel mio cuore".
"lo" and "la" here are not articles but pronouns; "lo" is masculine, "la" feminine, gender depends on the noun they're referring to, not on the subject.
Edit: I agree with erlampo "mio" is not necessary, but it put an emphasis that for a tattoo perhaps is appropriate. Just an opinion, though :)Help With English To Italian Translation?
"Lo porto nel cuore."
You don't need the "io". And you don't need "mio" because it's implicit.