Your answer is in the weapons chapter under Equipment, in the part about Impovised Weapons.
10ft increment, so you can throw it up to 10 ft. without penalty, with a maximum of 5 increments, so up to 50 ft. with a penalty of -8 (-2 for each 10ft after the first increment).
You'll score a critical at a natural 20 and do x2 damage, no matter the actuall threat range or critical multiplier the weapon has.
The damage depends on the size and general shape/use of the weapon, and most of them are on the GM's discretion.