I don't know what subject to do my Extended Essay in for International Baccalaureate?

I’m planning on doing my EE in Biology, but I have heard it is better to do it in English?

You can do you extended essay in any of the 6 subject areas that you are taking for your diploma.

The most popular subjects are English and History. However, this doesn’t necessarily mean it is better to do them in these subject areas.
You don’t only have to be able to answer the question, but you also have to be able to write at a university level.
However, in other fields such as the Sciences and Mathematics. Extended Essays are more experiment based and styled, but also assessed at a university level.

So when choosing what to do your extended essay in, it is important to consider what you area you are thinking about pursuing when you are at university.
If you are still unsure, but are thinking it is more likely going to be the sciences, rather than the arts, I would certainly recommend choosing the subject area which is going to be the most interesting for you.
This not only gives you a motivational advantage while writing your EE, but also is a major plus when applying to universities. If they are know you already know how to write, conduct and formalise research in the field you are applying for it certain programme, that’s certainly a plus!