Simple Explanations

Is there a website that offers simple explanations or tutorials for how to change the code for people who have minimal experience working with code? I tried to use the WP Codex doc about designing headers, and it says there’s certain code to look for under the Header template, but I’ve looked through the code and can’t find what it’s telling me is there (replace a jpg file, but I don’t see any jpg files). I have a little experience designing a website – nothing fancy – but this is not my forte, and I need basic explanations. I’ve completely lost every shred of patience I had using the WP doc.