If you die you end up in heaven. You can travel Snake Way and head to King Kai's or you can train. You can buy King Yamma's fruit for 300 credits at the check-in station which gets you to King Kai's 2 days faster. It takes 4 days to reach King Kai's from the check-in station normally. If you fight in heaven you gain the same amount of stats for winning/losing but you get no money or senzu beans. Only clothing comes with you to heaven. If you lose a battle in heaven, you're done for battles until you come back. I don't want people just staying dead forever.
King Kai is a master with a small planet that has heavy gravity. He can teach you every move there is, including specials. Training with him is like training in a gravitron upgrade that goes up to 21x gravity. If you have enough pl (7300) you can train for 25 stats a day. King Kai will only train you 17 days each time you die.