If I roll a higher number, can I decide to choose my opponents battlefield? And if so, does that mean that I get 2 shields, but that he gets the first action? I just picked up the starters this week and want to make sure I am playing correctly. Thanks!

You are correct on both counts.

If you pick your opponent's battlefield, you may distribute 2 shields to your characters as you wish, and your opponent will act first.

Thanks!