Can I donate blood if I was born with heart defect? (tga) I had surgery as a child, but I'm healthy now.

Yes. As long as you don't have other contraindications to donating blood, then simply being born with tga does not preclude you from donating. Please discuss this with your primary physician as there are numerous other contraindications.
Yes. So long as you're not anemic and not carrying a transmissible disease, engaged in high risk behaviors, or on certain medications, and generally healthy, you can donate. Bless you!
Talk to Red Cross. In all likelihood you received blood transfusions during your surgery. This will lead to antibodies in your blood stream this will make matching your blood more difficult for the blood bank, but does not mean you are ineligible. I would recommend trying the local red cross or allied organization and donating.