Using an improvised weapon (e.g. a chair, a frying pan, a big rock), default to 1d4+STR or 1d6+STR. Your GM may allow you to use bigger dice or other properties if it makes sense. An improvised weapon may be likely to break after a few hits, though!
Improvised Weapons
Core Rules v2 p. 34
Related Rules
3AttackCombat / Actions
Dual WieldingCharacters & Equipment / Weapons
Unarmed StrikesCharacters & Equipment / Weapons