By the standard rules in the players handbook at first level you get your full hit dice + con so your rogue's hit dice is a d8 so you get 8+1 = 9 hp for first level.
After first level you get the average of your hit dice rounded up + con.
So you get the average of 8 which is 4.5 and rounded up is 5 + your con of 1 which is 6. 6 + 9 = 15. That is how it is calculated.
It could be better explained in game. I hope that helps.
Last edited by unknown5; 08/10/20 12:24 AM.