If you want the user to be able to choose one list at a time, a simple HTML "SELECT" box could allow the user to set the "list" variable:
<select name=list>
<option value=1>List 1
<option value=2>List 2
</select>
For multiple list subscription, you can set the "lists" variable to contain an array of values, ie:
<input type=hidden name=lists[] value=1><!-- global list -->
<input type=checkbox name=lists[] value=2> Subscribe to list 2<br>
<input type=checkbox name=lists[] value=3> Subscribe to list 3<br>
You may also be able to use a multi-SELECT box as shown
here but I am not sure if when only one option is selected the lists variable becomes an array or a simple string, which could cause a processing problem if not rectified with JavaScript onSubmit()
More information on signup form variables can be viewed in the help
hereRegards