Choose a topic that:
1. really interests you. If nothing in biology interests you, do a different subject
2. is relevant to your future plans e.g. that could form part of a PS for uni or a job application
That should help you to decide that (for example) "I'm really interested in finding out more about behavioural ecology" or "It will be really helpful if I can show that I have an understanding of epigenetics"
The next stage will be to narrow down that topic area into a clear, well-defined question - one that is manageable within the timescales etc.