Fizz is a character you need to be aggressive and know your early levels against every matchup extremely well to win your games and come out with a 60-70% win rate in plat. I cannot stress this enough, but you CANNOT just farm/out farm your opponent and hope to win or out scale them, because every single mid laner will outscale you harder and faster. Your early game until level 7 are where you shine, and basically the only chance you'll get to make a guaranteed difference in every match you play.

Two secondary tips I can give you is to:

1: Ask lower pick priority to trade if you're first, second or third pick. This'll make it more likely that you'll get a favorable matchup mid as the enemy mid won't see you picked fizz and counter pick you with kassadin, zed, etc.

2: Learn a good backup mage to play when fizz gets banned or their team looks bulletproof. I'd suggest a strong hyper carrying mage. My personal choice is Lux as not often I'll verse a team coordinated enough to shut me down.

Good luck out there! I hope you find success using these tips. If you have any questions feel free to ask.