Create a script assertion with groovy script

This is a really vague question and one probably more suited to reading the SoapUI Guides.

That said, you have a number of of options, you can add a script assertion to the test step itself or you could create a Groovy test step to script one or many assertions.

Below is a really simplistic assertion that is bound to the test step itself....

import com.eviware.soapui.support.XmlHolder

// Grab the response from the step itself as XML
def holder = new XmlHolder( messageExchange.responseContentAsXml )

// You need to define the namepsace and you should be able to find this is in the reponse.
holder.namespaces["ns3"] = "urn:someorg:domain:1"

// Grab the node of interest from the XML Holder
def node = holder.getDomNode( "//ns3:customerInfo[1]/ns3:surname[1]" );

// Quick check to make sure the value exists
assert node != null

// Pull out the surname
def surname= holder.getNodeValue( "//ns3:customerInfo[1]/ns3:surname[1]" );

// Make your assertion
assert (surname.contains('Smith'));