best approach?

First time playing ATG 8, NL Only...100 cap....
Should I concentrate on pitching vs hitting? Or, what is the best approach for this type of league for a first time player?

Pitching is definitely an easier place to start. There are plenty of expensive, good starters that are similar in performance. If you grab 4 with good endurance and a decent relief pitcher, you can concentrate the rest of your money on hitters.

Then shape the hitters around your ballpark. There's a lot more to think about on the hitting side. It's hard to describe but you will get a sense of what works as you play the game.

I usually use 30-40% of my salary cap on pitching....but I've seen good teams with more and less spent on pitching.
Since it is NL only, I'm guessing that there is not a DH. If that is correct, your sp will get pulled for pinch hitters more often, so you might want to spend a little more on RP and look for a reasonable pinch hitting backup somewhere.

DH leagues are definitely easier to manage due to less uncertainty with the AI for pitching changes, pinch runners, pinch hitters, fielding switches, etc

So hard to give generic advice that really means anything. I would take a look at succesful teams on the Barnstormers page.