Players receive double their full Hit Points (HP) + CON (Constitution) modifier at first level. At each additional level players will receive Hit Points based on their Hit Dice (HD).
Each additional level characters receive 1/2 their HD + CON modifier in Hit Points. Your HD is determined by your class. It is the die type (d) you would normally roll to determine HP.
Wizard & Rogues HD d6 / 3 HP
Clerics HD d8/ 4 HP
Fighters HD d10/ 5 HP
Barbarians HD d12/ 6 HP