You need to start out with a topic. The topic should be one that interests you and one that relates to biology. There are a thousand topics that would be interesting for a seminar. Find one that grabs you. This is not a lifetime relationship.
Many students spend too much time hunting for the perfect topic.
There are hundreds of perfect topics for you so pick one and get going. If you can't think of a topic, use something to stimulate your thinking. Science News comes out each week and each issue has at least a dozen ideas worth pursuing. Check the library or biology faculty to borrow some copies. Beginning Just like a good book, your seminar preparation should have a beginning, a middle and an end.