You can use a third-party XML authoring tool like Altova's XMLSpy to generate XML schema from an XML instance document.
This procedure describes a simple way to generate schema from an instance, but is just a beginning. As with many wizard-style conversion tools, the results of this one are a "best guess" at getting the results you want. Keep in mind the following:
To Generate Schema from an Instance Document in XMLSpy
Once you have finished your schema, you can compile it to generated Java types. See How Do I: Use XML Schema in WebLogic Workshop? for more information.