The Linux Foundation's Intro to Linux course is a full, professionally-delivered course. Its usual price tag is $2,400 but is being offered for free from August 1st 2014.
For your money - or lack of money, in this case - you get over 40 to 60 hours of course work, taught by Dr Jerry Cooperstein, who oversees all training for the Linux Foundation.
To get the free course click on the words "Why do I have to pay? What if I want a free honor code certificate?" and the option to select the "Select Honor Code Certificate" option will show, which allows you to bypass the usual contribution fee.
Before taking the course it would be helpful to ensure you have Linux running on a computer (or virtual computer). If you have never done this before the Linux Foundation provides a helpful 16-page guide to get you up and going.
Sign up through edX and bolster your Linux skills. The course has no assumed knowledge except familiarity with computers and common software. If you can sign up for the course then you have as much experience as is required.
For the keen, you can register to get a certificate proving your completion and newly acquired Linux skills for $250.