Master degree and PhD in biomathematics
Yes there are MSc as well as PhD programs in Biomathematics. You should look for programs in Biomathematics and/or Mathematical Biology; this field of study is also branded as Systems Biology quite often. A quick Google search returns quite a few hits both in UK and US. The Society for Mathematical Biology has a good (but I would say somewhat US-centric) list of such programs available here.

Entry requirements usually depend on the program. Generally speaking good Maths and Stats (eg. Network/Graph Theory, Stochastics, PDEs) help a lot, as well as showing an interdisciplinary approach to your training. My best advice to prepare for such a course would be to be a great student. It is too early in your academic journey to focus exclusive in one area (eg. Stochastic Processes) and hope it will carry you through graduate school. Besides it will be very unlikely that a star-pupil in Maths will be denied entry to a graduate Biomathematics programme.