Compare the supported J2EE specs ( EJB, JMS, Webservices) of the Weblogic Version you are going to migrate from to the JBoss Server version. You will get the details from their official website.
Find out the architecture of the Weblogic Application Server ( How many servers? Clusters? Machines?) either from the config.xml or Admin Console.
Find the dependencies of the applications ( where are third part libraries?)
Find the resources needed by the Application ( Datasources / Queues & Topics / Resource Adapters)
Change the Proxy ( Webserver configurations)
Deployment descriptor changes
Does the application rely of Application Server security? If so you will have to create corresponding users in JBoss.
Migrate the Custom MBeans / Startup classes / Application Listeners.
Change the JNDI lookup code in the application.
Refer this link that explains in details.