How do I calculate my skill modifier?

I’m trying to fill out the skills section on my first character, but when I look at the character sheets in the Starter Set as examples of how to make a character, the skills that are circled have higher modifiers than their Dexterity or Strength or whatever should make them.

I’ve filled out all skills but those. What do I add to the ability modifier to get those numbers?

  • My abilities are Strength 13, Dexterity 14, Constitution 11, Intelligence 15, Wisdom 8, and Charisma 8.

  • Proficiencies are Arcana, Athletics, History, Investigation and Stealth.

  • My saving throws are Strength and Dexterity.

I just started D&D and have the Player’s Handbook, DM’s Guide, Monster Manual, and the Starter Set.