I think I would have a lesson at the beginning of the school year about citizenship in general and then on another day I would go into the concept of digital citizenship to make the connection between both concepts. I would explain that one pertains to the students' actions on the internet and other to students' actions in general. Furthermore, I would want students to gain a deeper understanding of three key points.
The first is the notion of cyber bullying and how it can cause serious harm to the person being bullied. Some kids have gone as far as to commit suicide because they were being cyberbullied.
The second notion about being a good digital citizen is that they need to make sure to use resources found on the internet wisely and document where they are getting their information. Just because something is out on the internet does not mean that they can steal the idea and not grant credit where credit is due. The other facet of this point would be to ensure that students know that not every single piece of information from the web is accurate and that they have to be discriminating when it comes to getting information from the internet(or any form of media for that matter) and take it at face value. For example, look at Fox News or the Weekly Standard.
The third concept I would want students to understand about digital citizenship is the notion of doing your part to keep yourself and those around you safe. Just because you can put all of your information out there on the web, does not make doing so a good idea. If they know any of their friends are doing things like displaying pictures or giving out their full names and address, they need to alert an adult because it could potentially lead to deadly situations. Students need to know that the internet can also be a dangerous environment, so they need to keep their wits about them and refuse to diclose private information with strangers just as they would in real life.