I have to do a 10-minute speech based on this:

Give an account of Britain’s colonial policies concerning Africa, Australia, India, and New Zealand in the 19th century.

I have little to no prior knowledge of this, so I'm not really sure where to start, or what I can even talk about - can anyone help?

The first thing you need to do is to learn about this subject. Check your text materials carefully. Take notes. Take more notes.

Then, you could Google Britain colonial policies 19th century.

After you've learned about this topic, decide on your thesis. What points support your thesis? Make an outline.

Then write the body of your speech.

Your last steps will be to write the introduction and conclusion.