HEAL
For Mid Katarina you can take Heal/Barier/Ignite.
You can take the heal if you are versus a squishy team it gives u heal if u are on low hp {or after Hourglas} or to catch somebody with the +Movement Speed.
BARIER
The Barier is very usefull for mid if u are versus Veigar/Zed/Vladimir or another one who has a ulti shot like Caitlyn to dodge it with your Barier and you can use your Barier for going under tower to kill E+W then when the tower atacks you use your Barier and your E again on minion or ward to escape and with your Barier you dodge the hit from the tower.
IGNITE
I use usualy the Ignite only if im playing versus a team who has Swain/Dr.Mundo/Volibear/Soraka or another healer to make his HP Rengen/Healing lower if no one other in my team has Ignite.

STARTING
For poke on the lane you can max W and use your combo Q+E+W so you can easy escape back with your +Movement Speed from your W or Max Q if you are scared to poke with your W.
For start take Q and on the second level take W or take your E if you are scared from ganks to you can easy escape.
LOSING LANE
When you get level 6 and you get your Ultimate so if you cant beat your enemy at mid go try to roam at bot to get some kills but then be squishy and take your Updated Boots couse you dont deal big damage or wait for your jungler to kill your enemy together.