Your damage is based on your attack roll.
Attack as normal, if you hit, then damage is done as shown below. So, if you need a 14, and roll 13, you miss. But if that hits, do as shown below.
20=double dmg
19=full dmg + double str dmg
18=full dmg + 1 + str dmg
17=full dmg
16=full dmg
15=full dmg
14=3/4 dmg
13=3/4 dmg
12=3/4 dmg
11=1/2 dmg
10=1/2 dmg (no str dmg)
9=1/2 dmg (no str dmg)
8=1/4 dmg (no str dmg)
7=1/4 dmg (no str dmg)
6=1/4 dmg (no str dmg)
5=minimal dmg (4 points of damage)
4=minimal dmg (3 points of damage)
3= minimal dmg (2 points of damage)
2= minimal damage (1 point of damage)
1=fumble!