I'd warm up the class by using something the students are familiar with, like, say, Netspeak symbols, the ones you see in text messaging or use on MSN.
Since the 2008 Summer Olympics are coming up (in China), you could talk about or ask the students what the various sports symbols mean. Then extend that into simple Chinese symbols, like mountain or river, or even further, the various writing symbols of some of the world's languages.
There are also road signs and other signs in around the school that you could talk about that students are familiar with.
Sign language uses symbols too, as do brand names. Now that is something they would definitely know something about.
The trick is to find a topic that they have some knowledge of so that they feel confident and willing to participate in the lesson.
Group work (brainstorming ideas), designing your own sign, matching signs to their meaning are some of the things you can do to involve the students. Remember, teacher talk time (TTT) should be a fraction of the amount of time your lesson plan provides for, so make sure your lesson has them doing more and you doing less.