At first level, all PCs get maximum HP.
After first level, players choose whether they will take average HP or roll for HP.
Average hit points are high on odd levels and low on even levels. For example: a fighter would get 10 HP (+con modifier) at first level, 5 HP second, 6 HP third, 5 HP four, etc. if he takes average.
Rolling HP is just like it sounds. Please roll in front of the gm or another player, and re-roll a 1 once. (If you roll a 1 twice in a row for HP, make better sacrifices to the dice gods next time.)