If the class you are in is not by itself an EJB you will have to use a JNDI lookup.
Context context = new InitialContext();
MySessionBean mbean = (MySessionBean) context.lookup("java:global/MySessionBean");
Make sure that JBoss published the EJB under the name you use.
EJB Singleton session bean from a java program
If you by any chance mean an external Java program, running outside of the JBoss AS' JVM, then you will need to employ an EJB client to connect to the AS and call methods remotely. Of course, injection will not work in this case and you will have to manually perform a lookup.
Thank you hd0815. Worked like a charm.