Starting hit points are based on your constitution score (not modifier), and you add in some numbers varying by class. If you want to go by level, here's the official excerpt from the Wotc-page.
Class Hit Points
Cleric (level × 5) + 7 + Constitution score
Fighter (level × 6) + 9 + Constitution score
Paladin (level × 6) + 9 + Constitution score
Ranger (level × 5) + 7 + Constitution score
Rogue (level × 5) + 7 + Constitution score
Warlock (level × 5) + 7 + Constitution score
Warlord (level × 5) + 7 + Constitution score
Wizard (level × 4) + 6 + Constitution score