HELP ME!!! What topic should I choose for the Speech?

Depends on your expertise, i would suggest common topic such as

1) 科學/科技 Technology. If science is your expertise, try research on some new technology. Nowadays, new technology should be abundance.
Example topic:
Wireless Network
Bluetooth
智能身份証

2) 新聞政治 News and politics. Current events are usually accepted speech topics.
Example Topic:
我自己唔係好留意新聞, 不過你打開報紙或上網睇, 一定會有岩心水

3) 社會問題 . 機本上, 任何城市論壇講緊/講過既野都可以攞黎講.
Here is a list of past topics: http://www.rthk.org.hk/rthk/program_archive.cgi


4) 哲學 Philosophy.
其實細心留意下你而家做緊既依個site: Yahoo! 知識, 你都會搵到好多好題材.
Here is a link to Yahoo! 知識> 藝術人文 > 哲學: http://hk.knowledge.yahoo.com/dir/?link=list&sid=102244094

So who will be the audience? What's the purpose of the speech? And what sorts of speech are you going to have? It's difficult to suggest if these answers are unknown...