How to learn biomathematics?
MathsBio is quite a large field. It is an interdisciplinary branch having utility in a lot of branches in biology like biophysics, biomedical, genetics and molecular biology. Applied  Mathematics is generally used in modelling and understanding biological phenomena where we have to deal with large amount of data, for example the use of graph theory for analysing biochemical networks. Systems biology is an emerging field which uses a lot of mathematics.

You should pick a book depending on the field in which you want to use mathematics, though having knowledge of elementary biology is necessary. I am suggesting some books which cover wide application of mathematics in biology. 

Mathematical Biology: I. An Introduction by JD Murray  

For systems biology:

Mathematical Modelling in Systems Biology: An Introduction by Brian Ingalls