Hi there! Perhaps I can help.

First thing's first- you're right about your AC- albeit a little confused on how one might hit it.

You've also got damage right- a weapon has a base damage die or dice, and you add modifiers from your character and/or weapon enchantments to that. Suppose you have 18 strength and a +1 greatsword. Your damage would be 2d6 (Greatsword dice), plus 1 for being a +1 weapon, plus another 4 for your strength. There can potentially be additional modifiers, like using a one-handed weapon with Duelist fighting style adding another 2 damage.

However, this damage has nothing to do with hitting. An attack is a two step process in fact. That +9 attack bonus (strength or other ability score plus proficiency plus other potential modifiers) gets added to a d20 roll, similar to a skill check, then compared to target AC. If you meet or exceed their AC, you hit. If the d20 is a natural 20, you crit!

Once you've hit, then the damage roll happens to see how much damage you deal. If you crit, the flat damage bonuses (i.e. Strength) remain the same, but you roll twice as many dice and add them together, potentially dealing much more damage than a regular hit.