Some of my coworkers were interested in learning unix, and so we created an internal mailing list where I could post regular, short lessons on unix along with simple challenges to apply what they learned. It worked out well enough that I wanted to make it available to my friends and family outside of work.
Since the challenges tend to build on one another, it's probably best to start from the beginning and solve the challenges before moving on to the later ones.
And let me know if I'm going too fast, but also poke around on the internet to find answers to things I don't explicitly explain. A big part of a new skill like this is knowing where to quickly get answers to your questions, so I'll try to give you the vocabulary for what to search for in the challenges.