21y/o starting to get into long term investing

I’ve just started to get into investing and I’m unsure of what to invest in. I’ve heard that EFTs are the way to go as they are a group of 50 - 500+ top companies in US/AU/NZ. Im just looking for some guidance on where to start, and a good “strategy” that some of you use yourselves. Also I was looking for some insight on where I should put my KiwiSaver. Any and all advice is welcome thank you all in advance