A good place to start would be a kde tutorial at KDE userbase for learning your way around kde. Mint is currently running 4.9 and the tutorials are 4.10 and very similar.
Also hold ALT and press F2 (ALT-F2) while in KDE and typing help wil show KDE help. ALT-F2 can be also used to run any program you wish. For example ALT-F2 and type konsole will open up a the Konsole terminal.
Once you want to go further than the desk top you could try linux.org to learn Linux basics. There is a beginners guide here.