WBMFAG - Armour Class Help!

Hey guys, trying to come to grips with WBMFAG (playing solo, so no one around to help) and struggling get my head around the Descending Armour Class (DAC) system for White Box Fantasy Adventure Game (is it THAC0 or is it Target 20??).

Let's say I roll a level 1 Human Fighter with a +1 in strength and I was fighting a human bandit with an AC of 7.

To my understanding I would need an 11 or higher to hit?

I'm finding it really hard to understand the armour class system as it relates to equipment. For my level 1 fighter, I have bought a set of chainmail (-4) and a shield (-1) which is a total Armour Class of -5 (apparently helmets are for show?). To my understanding that would require a 13 to hit?

I have no idea if i'm doing this right or not, its very confusing to me. Any help or advice would be appreciated!