"No, I am Žofie."
Translation:Ne, já jsem Žofie.
The first one is used to correct someone who thought that Žofie is someone else when it is, in fact, you.
The other one is used to correct someone who thought you are named differently. At least in the default neutral-stress pronunciation, you can stress Já in the other sentence as well.
Remember, the last piece of the sentence is the new most important information by default.