Grails Unit test controller

So im trying to do a Unit test on this controller, I'm new to this sort of testing so I dont know from where do I start and how to test things that are needed to be tested, like user, render view.)

class ShoppingCartController {


def shoppingCartWebshopService
def shoppingCartService
def springSecurityService
def checkoutOperaService
def databaseService
def dodAdresaService


def payingOrder(SetShop1Command cmd) {

    def user = springSecurityService.isLoggedIn() ? springSecurityService.currentUser.sifpartnera : null
    def tempAdrese = databaseService.getAdrese(user)
    def adrese = []
    tempAdrese.each{
        adrese.add(it.adresa)
    }


    if (cmd.hasErrors()) {
        render(view: 'OrderChoseAddress', model: [cmd : cmd, params : params, adrese: adrese ])
    }



    def inputUnAdrese = params.inputUnAdrese

    [inputUnAdrese: inputUnAdrese, adrese: adrese]
}

This is my test

@Mock([SpringSecurityService,DatabaseService])
@TestFor(ShoppingCartController)
class ShoppingCartControllerTests {

void testPayingOrder(){

            when:             
//          params.user = 19        
//          user.equals(id)
//          println user
//          controller.OrderChoseAddress()
            controller.params.user = "admin"
            controller.databaseService = new DatabaseService()

            then:            
            assert model.adrese != null 
        }