If you're really starting from nothing and want the friendliest (i.e. the least terse) introduction then I might suggest the highly readable PDF version of that PLINK 1 documentation:
I think it's useful to understand what is in a PED file, since it's informationally equivalent to a BED file, I believe.
And I think it's useful to see the historical origins of PLINK1, as it may give you a better grasp of how PLINK 1.9/2 have built upon that legacy.
But Christopher will please correct me if I'm giving bad advice!