Katarina is one of the most powerful characters in the game if she is in good hands. All you need to play well on it is not to get controlled. If you think you'll fall under control, you'll lose, since Katarina's health reserves are very small, and I don't recommend the tank build. You can watch the combos on YouTube, they are quite simple and easy to remember. For example E-W-Q-R or just E-W-Q if there is no ult. You can also use E-Flash to instantly reach the enemy over a long distance. :')

A lot of Katarina players use the AP build, but I don’t really like it because it’s weak against potential tanks. Imagine that you have 4 weak players on your team, and your opponents have 2+ tanks. This will be quite a difficult matchup for AP Katarina because she is designed to kill squishy champs, and AD Katarina's build includes at least one anti-tank item that literally helps take tons of damage off them. My build was tested against a dummy with 8k HP, 400 resists and 400 armor, he literally loses all his health in a couple of milliseconds. However, then, after doing a few more experiments, I began to use the hybrid build instead of AD, as it showed more impressive results.

In general, all that can help you is training, practice, watching videos on YouTube, montages, edits, additional guides, as well as watching Twitch streams from popular streamers (for example Katevolved). Good luck, young Katarina's mainers!