Hephaestus was the product of Hera's jealousy. You see, Hera was bitter because Zeus had born both Dionysus and Athena without their mother's bodies present for the births. Remember, Athena sprang from her dad's head, and Dionysus was born out of his thigh. So Hera decided to get even and ate some magic roots that fertilized her. Then she went about bearing Hephaestus. Once he was born, however, there were problems.
Hephaestus was seriously lame and awkward. He couldn't walk. But how this came about has two possibilities. Some say that he was born that way - deformed because there was no man involved. Some say, and this is more accepted, that Zeus and Hera got in a huge fight, and brawny Hephaestus tried to appease both parties, but sided more with Hera. Zeus, furious, hurled the young Hephaestus from Olympus. The Smith God fell for a full day before reaching the ocean. He was alive - immortal as he was - but his legs were destroyed.
But the story doesn't really end there. I mean, Heffy was just a kid - how was he supposed to get back up to Olympus? Well, luckily for him, lots of Nymphs happened to be around - specifically Thetis and Eurynome. These two nymphs are two of the most powerful sea goddesses there are - so he was very lucky. They kept him in their underwater grotto, and he set up his first forge and made them lots of really pretty jewelry. Then, one day, Thetis was chillin' with Hera while wearing this brooch that Heffy had made for her. Hera, who was a sucker for a pretty bauble, forced out of her who the craftsman was. When Hera found out it was her own son she immediately brought him up to Olympus and set him up in a forge with twenty bellows working day and night. Not too shabby!