You have 6 stats, Constitution is one of them. Your class dictates what size is die you use for HP on level up.
So a fighter uses a d10 so at level 1 he’d get the max of 10 plus constitution. Let’s say 3 since he’s thicc. So he starts with 13.
Then every time you level up you roll your hit die and add your constitution. So at level 2 the fighter rolls 1d10, let’s say he rolled a 7. Then add constitution of 3 for a total of 10. So at level 2 the fighter will have 23 hp.