Start with LinuxCommand.org.
A good cheatsheet will go a long way. Google is your friend. One great option is "Linux Gems", try going through and learning about each one.
If you know DOS at all, Linux for DOS users might help. (It also works in reverse if you know Linux and you're trying to survive on Windows.)
When you don't know what a command means, a nifty little utility called Explain Shell can help, and from there you can read the man or info pages to glean more.
For book people, I recommend The Linux Command Line by William Shotts, from No Starch Press.
Start with try.github.io
Once you feel comfortable and would like to move on to some more meatier Git concepts, try Atlassian Git Tutorials where you will become a Git Guru.
GitHub Guides are really great for getting you used to the flow of Git and how to really start contributing to projects.
For a fuller understanding, the best thing to do next is simply read the GIT manual
If you're having trouble along the way, First Aid Git may be able to help you out.
- See also: DVCS and DAGs Part 1 and Part 2 (from ericsink.com)
To learn when and what type of questions to ask, when you are looking for help, see What have you tried?
Being able to troubleshoot problems quickly is a very important skill. There is a process you can use to help you troubleshoot quickly, even if you don't know a lot of details about what you are trying to fix. Steve Litt explains this in Troubleshooting: Just the Facts. The website isn't anything flashy or impressive, but the information is a gem.
Also by Steve Litt, Rapid Learning for the 21st Century can helping you to accelerate learning anything, by using a structured self-learning approach. The First 20 Hours by Josh Kaufman is another resource in this area.