Standard Jaina build

lvl1: Fingers of Frost  for mana regen. Because a mage needs mana... and Jaina.. You know, she's a mage and.. abilities need mana

lvl4 & lvl7: Frost Shards   and  Ice Lance  have a good synergy. When lvl 7, try to hit your enemy with an other ability before using  Frostbolt . Q is your main weapon for "substain damage"

lvl10: You have the choice for lvl 10. Yes really. Team with lack of CC ? Go  Ring of Frost  . The enemy team has a ninja or child with duo guns ?  Summon Water Elemental  is good for substain damage and force them to fly away (Mada mada)

lvl13: You don't have enough confidence of yourself or your team (or both?). Take Ice Barrier  for a frosty shield. But don't forget to hit your enemy to gain that shield. Otherwise, you are a strong and independent mage ? You want to (breakfree?) kill the enemy team and die with them ? Perfect !   Icy Veins  is made for you. Breath in, breath out, hit  Icy Veins  and spam all of your abilities like hell ! I mean a frosty hell of course.

lvl16: For lvl 16 it depends. Your team has good damage output and good CC ? Go  Northern Exposure . Lot's of enemy melee heroes ?  Numbing Blast . For example The Butcher is charging you ? Wait a bit > Blizzard  on your feet > Ice block if you have it to avoid stun > Cone of Cold  > Frostbolt  > Step back > Dance > ??? > Profit

lvl20: Do you have mobility ? No you don't.  Ice Blink  is good to escape.