Basic Computer Skills Curriculum - This is a good place to start if you are teaching students with little or no knowledge of computers. This is for PC users; there seems to be nothing for Mac users. This is from the St. Paul Literacy Consortium.
Digital Literacy - Resources from the US government for teaching computer use. There are very basic guides for new computer users, and advanced topics which could be used with middle and high school students.
Computer and Internet Lesson Plans - From Hot Chalk. There are a lot of these so it would be a good idea to use the search box to locate the grades you are interested in.
Read Write Think Lesson Plans - There are many technology and internet related lesson plans here. Use the search box to locate appropriate plans for your class.
Tutorial on Computer Basics - This is a tutorial for those who know nothing about computers, cell phones, tablets or the Internet. From LearnFree.org. Note that LearnFree has other technology tutorials such as one for Windows.
Open Educational Resources - These are freely available resources from all over the world. This Crestive Commons site tells you about these resources and tells you how to get started. Note the PDF which tells you about them, and the getting started section at the bottom which has links to k-12 lesson plans.