Re: Where should I start
As said, start with the first episode of the new NXT, although you could quite easily start watching this week and pick up most of the characters and gimmicks pretty quickly.
If you're interested in The Shield, only Rollins of the three has been featured prominently on NXT. Roman Reigns was involved in a couple of short squash matches and promos but nothing major, and Ambrose wasn't featured at all on televised broadcasts.
x78 is a one note gimmick don't engage him.