See the dedicated non-scientists section.
Technically you only need an understanding of basic matrix algebra and complex numbers, but I would recommend being very comfortable with these topics as they form the basis for almost everything we cover. For certain algorithms other maths will be needed such as the Fourier transform, but unlike matrix algebra and complex numbers this is not needed for everything.
There is a preliminary mathematics section that covers the matrix algebra we need but this should be considered as a recap or check-list to make sure you have the understanding and it is fresh in your mind. If you are unsure, then check over the preliminary mathematics section to make sure you're familiar with each topic.
If you’ve already studied quantum physics I’d recommend starting at preliminary computer science then skipping straight into quantum computing. If you have any issues you can look back at matrix algebra to refresh your memory.
You should be able to head straight into the quantum computing sections. If you need to remind yourself about complexity classes or matrix algebra you can read over these in their background sections.