I know this is an old question, but I know there are still people with this same question(I did up until today)
If you add a new activity to your manifest file, there's a special link to click on to automatically create the new Activity, complete with the onCreate() method ready to be filled in.
Open the AndroidManifest.xml, and go to the 'Application' tab. Under 'Application Nodes', find and click the 'Add' button. You will likely create a new element at the top level, so select that option, highlight 'Activity', and press OK.
Once you've created the Activity, go to the 'Attributes for Activity' and fill in the name. Once you've filled in the name you want, click on the blue 'Name*' link next to the field. The new file wizard will show up, and all you have to do is press OK.
Voila! New Activity, registered in the manifest and as a ready-to-go Java class.
