A few years ago, I made a spreadsheet that calculates the average damage of an attack, taking attack bonus, average damage roll, damage dice that aren't multiplied in a crit, target's AC, threat range, and critical multiplier into account.
Assuming you've got a Strength 16 fighter using a non-masterwork greatsword, here're average damage stats against AC 13, 15, and 17 enemies for a character with neither Power Attack nor Weapon Focus, for a character with Weapon Focus, and for a character Power Attacking for 1 point.
None --
AC 13: 7.26
AC 15: 6.05
AC 17: 4.84
Weapon Focus --
AC 13: 7.865
AC 15: 6.655
AC 17: 5.445
Power Attacking for 1 --
AC 13: 7.26
AC 15: 5.94
Ac 17: 4.62
The correct choice for optimizing your damage output at 1st level is Weapon Focus. In fact, Power Attack is gonna do nothing but hurt your effectiveness. I'd wait to pick it up until you've got a significant portion of your attack bonus that you can convert to damage, and attack bonuses start to outrun AC values.