Izotope have written a free guide to mixing for beginners.
The 69 page 8.8 MB Adobe pdf guide, starts off with an introduction to what mixing is. It tells about the four elements of mixing, which are according to Izotope: Level, EQ, Panning and time-based effects.
The guide then dives into each of these elements with a dedicated chapter followed by chapters on Distortion, Reverb, how to approach a mix and how to deliver your final mix. It finishes with some tips from professional mix engineers like Joel Hamilton (Elvis Costello, Pretty Lights, Matisyahu), Jimbo Barton (Rush, Queensryche, Matchbox Twenty) and Ted Paduck (Nine Inch Nails, Aerosmith, Run DMC, Busta Rhymes).
The guide of course has a strong focus on Izoptope plug-ins but it is well written and has plenty of good information and tips for beginning mixers. You can download the Mixing Guide for free from Izotope’s website.