You'll need to create a runnable class that extends thread. (Note, there may be other ways to do this, but this one works.
So you'll need something like
public class BackgroundTask extends Thread{
private Object _screen;
public BackgroundTask()
{
}
/**
* Implementation of Thread.
*/
public void run()
{
//Do some background task
}
Now from your main screen, you simply need to call it.
//Start my background task
new BackgroundTask().start()
Start is a method inherited from the parent Thread class, so it'll take care of spawning a thread for you.
Hope this helps.