I want to start speedrunning amazing mirror, any tips? This is going to be my first time speedrunning a game btw

Best category to start off with is any%. There are many alternatives if you don't want to do any of the mixes by SK https://www.speedrun.com/katam/guide/p99j6. I suggest trying to at least learn the wheel mix at the beginning, even if you don't get it first try, keep exiting in and out of the door for multiple tries. However, using fire is just fine it does not lose that much time optimally compared to wheel until you try to go for sub 30. If you have any more questions about the game, feel free to join the Kirby speedrunning discord https://discord.gg/N8NF2rm.