Let's say a fighter gets a +1 to any ability every 5 levels and puts each point toward strength.
For simplicity, let's ignore the magic, and other bonuses because we haven't really been told what those bonuses are yet.
(Page 3 of How to Play)
That's a possible strength score of 19(+4) for a 5th level fighter and a 24(+7) for a 30th level fighter.
So a 30th lvl fighter attacks at 1d20+7 doing 1d8+7 damage and a 5th lvl fighter attacks at 1d20+4 doing 1d8+4 damage.
This also means that every fighter will always spend his ability level-up point on strength to aid his strength based attacks.