In recent years the range and quality of web-based training has improved enormously. In fact it's easy to become overawed by the sheer number of choices available for budding coders and computer science teachers.
One piece of advice is to stay clear of sites that employ novelty languages. While they might seem a little easier at first, all too soon you'll hit a dead end. Far better to select websites that base their content around well-known languages. In this way you'll benefit from a wider set of learning materials and have the support of an active coding community.
A website that acts a little like a coding portal is code.org. The collection of information and links have a quality feel and the scope encompasses both pupils and tutors.
If you have a younger audience in mind then take a look at The Hello World Program. Here puppets are used to used to convey key coding concepts in a memorable and interactive manner.
Khan Academy is another extremely popular educational website. Here the scope is much wider than just programming, with topics surrounding area such as Maths, Science, Economics and Humanities. Khan's computer science studies are centred around the Python scripting language. While the Discovery Lab element has a practical hands-on approach to learning, with a number of interesting robotic elements.
Does the sheer volume of online web tutorial material seems a little daunting? Then why not visit the World Wide Web Consortium (W3C) website's tutorial page. As the W3C is responsible for defining the HTML, CSS and XML standards you can rely on the validity of the information presented.