When I roll the die on my turn to attack and roll 15 does that mean I do 15 damage? It confuses me because I see most people's characters only have around 10 hit points which would mean majority of the time they will just die.
No, rolling a 15 is not 15 damage.
An attack and its damage in D&D is (since at least 3rd edition!) handled in two steps:
- Roll 1d20 + Attack Bonus (depends on your edition, class and stats) + situation modifiers (including penalties)
- If you roll the targets Armor Class (AC) or higher, goto 2, otherwise goto 3
- Check your weapon's damage: roll that die, possibly add stat modifiers.
- That was your action. Check if you got another, otherwise you are finished.