Getting started with Google App Engine for Java is easy - downloading the SDK and deploying your first demo app can all be done in a few commands.
Assuming you've already created an app-id using the Google App Engine web interface, getting started is as easy as ...
# Download and unzip the SDK
curl -O http://googleappengine.googlecode.com/files/appengine-java-sdk-1.2.0.zip
# Start the local test server
# Now browse to http://localhost:8080/ and make sure the app works.
# Try editing war/guestbook.jsp and make a few changes.
# Add your app-id to appengine-web.xml: <application>your-app-id</application>
../../bin/appcfg.sh update war/
Now your applications should be deployed and ready for requests at http://your-app-id.appspot.com/.