I did it from memory, myself.

The three stats you can now boost are Hit Points, Stamina and Magic Power. Don't worry about boosting your Hit Points, as when you get to Level 99, you'll be close to 9999 HP with everyone anyway. Stamina defends against instant death attacks and determines how many HP you recover per turn with the Regen status. Magic Power is the raw damage stat for your magic, and it also determines how many HP you will recover when casting healing magic.

If you're going to boost anything now, I'd boost Magic Power, but don't go crazy, for as you can see, there are still many Espers to get.