Hit die

The type of Hit Die used by characters of the class determines the number of hit points gained per level.

HD Type Class d4 - Sorcerer, wizard d6 - Bard, rogue d8 - Cleric, druid, monk, ranger d10 - Fighter, paladin d12 - Barbarian

A character rolls one Hit Die each time he or she gains a new level, then applies any Constitution modifier to the roll, and dds the result to his or her hit point total. Thus, a character has the same number of Hit Dice as levels. For his or her first Hit Die, a 1st-level character gets the maximum hit points rather than rolling (although Constitution modifiers, positive or negative, still apply).

For example, Vadania gets a d8 Hit Die because she’s a druid. At 1st level, she gets 8 hit points instead of rolling. Since she has a Constitution score of 13, she applies a +1 bonus, raising her hit points to 9. When she reaches 2nd level (and every level thereafter), Vadania’s player rolls a d8, adds 1 (for her Constitution bonus), and then adds the total to Vadania’s hit points.

If your character has a Constitution penalty and gets a result of 0 or lower after the penalty is applied to the Hit Die roll, ignore the roll and add 1 to your character’s hit point total anyway. It is not possible to lose hit points (or not receive any) when gaining a level, even for a character with a rotten Constitution score.