@djava: AC 11 is rather low. Base AC is 10 so yeah, his AC and his sister's are minimal, tbh 5% difference. To succeed hitting someone, you need to roll a 20-sided dice , add your personal bonus ( depend on class + other things. For a lvl 1 character , bonus is normally +5 +6) and compare to target AC. So if bonus of attacker is +5, you have to roll 6 or above to hit MC ( so 75%). A normal Dnd character in lvl 1 is expected to have AC 12-13 for spell caster, AC 15 - 16 for fighter or hunter, AC 17-19 for tank.
Goblin attack is 1d4-1 means you rolls a 4-face dice to determine the damage and reduced by 1. So goblin rolls 4-sided dice, it get 1. -1 so it hit the attack but doesn't deal any damage. But if the goblin rolls 4, it will deal 3 damage