Start with your base Armor Class from your armor, if you are wearing any
Add your Dexterity modifier, up to the armor’s maximum if it has one
Add any shield bonus if you are using a shield
Add any magical or item bonuses to AC
Add any class, feat, spell, or feature bonuses that increase AC
Add any situational bonuses that apply
Apply any penalties or reductions that affect AC
Use the final total as your Armor Class