Roadmap to learn CNN in tensorflow from scratch
I'm assuming you're talking about medical images classification, rather than localization.

Personally I recommend Kaggle, it has an awesome forum and people share their codes and opinions there.

You can start at Digit Recognizer, it's actually the well-known MNIST dataset(hand-written numbers). There's no relationship between MNIST and medical field. However there are some common techniques and tricks as they are both image recognition/classification problems.

If you encounter problems, read the other ones' codes at the kernel section, especially those with upvotes.

In the meanwhile, comment section is a good place to learn. I learned a lot there when I started to learn CNN.