Sounds like you’re using an old character sheet from 3e (fortitude, reflex, and will haven’t been stats in the game for about 15 years). Make sure you’ve got one for 5e. Even without a rulebook, you can still use the free basic rules to create a character.

Base attack is determined by adding your proficiency bonus (if using a weapon you’re proficient with) plus either your Strength modifier (for melee attacks) or Dexterity modifier (for ranged attacks). Add this combined number to a d20 roll and compare the result to your opponent’s AC. If you match it or beat it, you hit.