How do I prepare for Paper 1? I'm really worried because there's so many words I don't know

So Paper 1 is reading comprehension and, unlike at GCSE, the source texts are taken from real life from the world francophone. This will mean that you'll find words, phrases or constructions that you might not know at all or aren't familiar with - this is absolutely fine! There are three ways to ideally tackle Paper 1:
1) have practised and read a lot in French outside of class,
2) use the context of the sentence and paragraph to help guide your understanding, and
3) sometimes, the question and the optional answers can clarify your misunderstandings too!
Another very important fact to bear in mind: no one is perfectly fluent in ANY language, for the top grade you don't have to know the French dictionary cover to cover! But, what this IB paper is looking for, is the skill of a linguist to not necessarily have seen and learned a word before but still try to work out some meaning from it.