When I was showing my wife's mother (in her 80s) the internet I did as follows.
I asked her to name a film star or singer she liked in the past, and she chose Doris Day.
I then went into Google and searched under Doris Day and then showed her some of the sites.
She was able to see what films she had been in, see pictures of her at different ages of her life, read details of her life, and we even found a site where we could listen to parts of her songs.
I operated the keyboard and mouse, and did not try to explain everything I did, but after that she had a better understanding of what the internet was.
Another option is to let them play some of the simple games that are supplied with Windows, like Solitaire or Minesweeper.
These are simple games but it will give them a basic idea of how to use the keyboard and mouse.
The key is to keep it simple at first, dont try to rush them, and remember to explain very basic things like how the keyboard and mouse works, how you open and close programs and so on.
Just take it slowly, and after you have shown them the basics, let them learn by doing it themselves (with you watching).
You can also buy books with titles like "Computers for seniors" and things like that (Smiths or Waterstones will have them).