Prerequisites: You must have at least 13 Strength or Dexterity. You must be at least 5th level. You must have at least three character levels in one of the following classes: Bard, Cleric, Druid, Fighter, Paladin, Ranger, Rogue, Warlock. You cannot take this feat multiple times, nor can you select a Fighting Style twice.
You have come to specialize in one facet of combat and weapon use. You may select one of the following benefits.

  • If you have at least three levels in Fighter, Paladin, or Ranger, you may select one additional Fighting Style from your class's list.

  • If you have at least three levels in Bard, you may select from the Archery, Dueling, or Two-Weapon Fighting fighting styles.

  • If you have at least three levels in Cleric, you may select from the Defense, Dueling, or Protection fighting styles.

  • If you have at least three levels in Druid, you may select from the Great Weapon Fighting, Protection, or Two-Weapon Fighting fighting styles.

  • If you have at least three levels in Rogue, you may select from the Archery, Defense, Dueling, or Two-Weapon Fighting fighting styles.

  • If you have at least three levels in Warlock, you may select from the Defense, Dueling, or Great Weapon Fighting fighting styles

Taken directly from d&d wiki (https://www.dandwiki.com/wiki/Combat_Style_%285e_Feat%29)