Hi my name is tony, i played league right now for 2 years, played like 600 games with katarina. I started playing her on mid with as an ap carry, but soon i learned that in ranked game u will be usually get focused a lot and you wont do as much damage as u can.
But my top lane katarina is way much better that the ap carry one. First of all u will solve the survivality problem, but still doing solid damage and will be realy usefull for your team.

Teleport is the most viable pick for top katarina, grants you huge map presence, you can teleport for ganking bot lane, going b and return without losing cs and much more things. Even knowing this you still wanna pick any other summoner spell, ignite is also viable.

Flash is always a must on this kinda of champions, flash + shumpo can help chase enemies with low hp, flash can be used for repositioning in teamfights, jump walls and it will save your *** so much times! But try to save flash for when you dont have shumpo or there are not any minion or ward near.

I picked w as first skill just because u will be playing mostly against melee champions on top lane, w is just great for some harass, making them fear to get close to get some cs and also helps you to farm better. Then i usualy pick shumpo just because top is ganked a lot and maybe at lvl 2 you will need to escape.

The core build: Sorcerer boots (you deal only magic damage so its almost a must, also synergizes well with sunfire cap), Sunfire cap (just great item on katarina, it gives her tankyness and aoe dmg with one synergizes so much with her skills), Rylai (will give you much more utility, your team will thank you for that slows and you will be pleased to see how people tries escaping you without any posibility just because of your w speed buff, slows and move speed runes)

Usually i go the most tanky build but i let you a more damage focused build.

Start items usualy should be: boots 2 pots and 1 ward, if you are against a hard ad start with cloth armor and 5 pots.