Your primary class determines your hit die size.
Average uses floor(die/2)+1 per level after 1st. Maximum uses the full hit die value every level (often used in home games).
If you multiclass, select your second class. Multiclass levels use their own hit die, not the primary class die.
The Tough feat adds +2 HP per character level (retroactive to all levels).
Hill Dwarves gain +1 HP per character level from their Dwarven Toughness trait.
Draconic Bloodline sorcerers gain +1 HP per sorcerer level from Dragon Resilience (Sorcerer subclass feature).