If you're a 1st edition-style PC, you die. No save.
If you're a 2nd edition-style PC, you get to make a saving throw. If you fail, you die. If you succeed... never mind, you're not going to succeed.
If you're a 3rd edition-style PC, you get to make a Swim check. If you fail, you take 20d6 damage. If you succeed, you take half damage and Swim out of the lava.
If you're a 4th edition-style PC, you get to make a saving throw. If you fail, you take 5 ongoing fire damage. You may exit the lava pool as a minor action.