Your heart is filled with sin? Not if it’s new!

How do I share the message of God‘s grace with my father without it becoming a debate? How do the Jehovah’s Witnesses differ from born-again believers? At salvation, are we even forgiven for our future sins? Somebody told me that when I sin, my heart is filled with sin and I need to cleanse it. Is this true? If not, what is the healthy response to sinning?