Hello,
you should:
1) define the parameter in you report definition (.jrxml file; in <jasperReport>...</jasperReport>
F.e.:
<jasperReport>
......
<parameter name="userName" class="java.lang.String">
</parameter>
....
</jasperReport>
2) in your .java, where you compile and fill your JasperReport, you should create a Map of all parameters and pass it as a parameter to the FillManager
F.e.:
Map parameters = new HashMap();
parameters.put("userName", userName);
JasperPrint jasperPrint = JasperFillManager.fillReport(fileName, parameters, getConnection());