Well first level gives you max hp. You can either roll the dice for levels 2-5. Or you can take the majority for them. Under every class you see something like 1d10 (6) + con mod. The number gives you an average of just over 1/2 of the possible max. So you can take the number in the parinthasese or roll and take whatever you roll. Regardless of which you take, you always add your con modifier.