To start a new topic, send a completely new message to the group via email and be sure to use a subject line in your email that has not been used before in the group. Messages are grouped in topics primarily based on the subject of the message. So if you're trying to start a new topic but are using the same subject line as a previous topic in the group, your messages will be added to this topic rather than starting a new one.
If you are trying to start a new topic by clicking "reply" to an existing message from within your email client and then changing the subject line, the Groups system may still group your message with an existing topic based on the "thread id" that appears in your email header.