How will I know I'm saying something right if nobody corrects me?

The magic of learning with comprehensible input is that you gain the same kind of intuitive feeling for what’s correct and what’s not that native speakers have, without anybody having to explain it to you. Once you have acquired enough of the language, you will be able to produce sentences in the language because you will know that that’s how they should be said. Besides having experienced this ourselves, there is a lot of research (read any of the meta-analyses by John Truscott) showing us that both spoken and written corrections don’t work, since they don’t have any lasting effects on acquisition. This even holds true for children learning their first language. There is a huge amount of variation in the amount of corrections that children get by the adults in their life. Some don’t get any at all. Still, they all get to become native speakers of the language.