How do you know what you are reading in Japanese?

A lot of practice, a lot of lessons, a lot of reading…

Try break it down word for word. Perhaps you know the word but you don’t recognise it in the sentence, or maybe you don’t know the word, in which case you should figure out what it means.

I mean, you can always just use google translate/deepl to get an idea of what it’s saying. Otherwise if you’re actually asking for beginner books there’s genki/minna no nihongo and probs a whole bunch of others I don’t know. Someone more knowledgable than me can advise you on that area. If you’re interested you can take a look at the resources thread the community has compiled. That might help give you a better idea of what’s available The Ultimate Additional Japanese Resources List!