Play safe, let first wave push into you, freeze, poke opponent with q, use q e w AA e R combo as a beginner combo, easy lane build dark seal 3 ppts, hard lane build cloth 4 pots or long sword 3 pots, easy lane take ignite, hard lane take barrier or if new to yasuo matchup take exhaust, last hit with autos or q or dagger pick up, Zed is your easiest lane match up, lux or xerath would be your hardest, don't all in midlane bruisers like Aatrox or Rumble or Mordekaiser instead take TP and apply global pressure.
If your grinding S tokens then take TP and ignite, farm lane perfectly, go 21 / 0 then ignore the game and your team and TP to catch big minion waves or pick up a double kill. Farm the jungle and maintain above 6.6 cs per minute for an S - rank or higher, and start longsword and 3 pots if you will be in a super push matchup where you will have to last hit undertower.
Most importantly, don't play Katarina in ranked. She brings nothing but damage and kill steal tilting from the team. Cant duel, splitpush, apply cc, powerfarm, global ult, mediocre waveclear without a few items. Can't carry a team in low elo since players don't know how to play around her strengths.
Reference: M7 Kat and M7 Riven player...
Go watch Katevolved or Katlife and notice how they farm, roam, engage, select opponent specific runes, track the enemy jungler, ward, overstay, manage waves, communicate and work with their team, disengage, find picks and much more for their team.
Sorry for the long read, hard stuck Iron Player who is willing to help you out if you have any more questions...