In every life you perform actions and thus create consequences. Right now you live a life that is shaped by consequences of your past lives. Due to karmic cycles, the same conditions come into your life, and since you do not care for your past lives and your past actions, you keep repeating the same mistakes. Thus you keep up the cycles and keep being born into Samsara.
But then how do I know my next life won't just ruin it all?
I could be born into such a different culture that I wouldn't even be aware of my actions and consequence.
I'm quite new to Buddhism so apologies if my question is silly.
This. The topic is quite complex, but even if it is true there is no self that will be reborn in another life, there still is a kind of "consciousness" that is shaped by karma and that will get rebirth. This of course if you don't escape the cycle or bow to be reborn in a Pure Land.