Hi,

First of all, please go through a few documents explaining the need/way for a test class.

A few links that might help:

http://www.forcetree.com/2011/06/guide-to-writing-test-methods-for-apex.html

http://wiki.developerforce.com/page/An_Introduction_to_Apex_Code_Test_Methods

http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_qs_test.htm

For the above class, you can use the below test coverage class:

@isTest
public class MyTestClass{
	private static void test_UserAccountCtrl(){
		// create data
		// in your class, we are accessing user and contact data 
		
		// case 1 - for a non-portal user that doesn't have a Contact
		UserAccountCtrl controller = new UserAccountCtrl();
		controller.UserAccountCtrl();
		
		// add other required fields for Account if any
		Account a = new Account();
		a.Name = 'testAccount';
		insert a;
		
		// add other required fields for Contact if any
		Contact c = new Contact();
		c.LastName = 'testing';
		c.AccountId = a.Id;
		insert c;
		
		
		Id ProfileId = [Select Id From Profile Where Name = 'YOUR PORTAL PROFILE NAME' LIMIT 1].Id;
		// case 2 - for a portal user that has a Contact
		 User u = new User(alias = 'standt', email='test@test.com',
		emailencodingkey='UTF-8', lastname='lastname', languagelocalekey='en_US',
		localesidkey='en_US', profileid = ProfileId, contactId=c.Id,
		timezonesidkey='America/Los_Angeles', username='username@test.com');
		
		insert u;
		
		// now run the below code as a portal user, here you'll get the contact id for the user
		system.runAs(u){
			controller = new UserAccountCtrl();
			controller.UserAccountCtrl();
		}

	}
}

 Let me know if any questions/issues. Thanks!