testComp.cmp

<aura:component implements="forceCommunity:availableForAllPageTypes,flexipage:availableForAllPageTypes,force:hasRecordId" controller="trTestCtrl" access="global">
<aura:handler name="init" value="{!this}" action="{!c.doInit}" />
<aura:attribute name="testStr" type="String" default="default value" />

<div>{!v.testStr}</div>
</aura:component>

testCompController.js

({
	doInit : function(component, event, helper) {
    var action = component.get("c.getTest");
        action.setCallback(this, function(response){
            var state = response.getState();
            if(state === "SUCCESS"){
                var str = response.getReturnValue();
                component.set('v.testStr', str);
            }
        });
        $A.enqueueAction(action);
	}
})

trTestCtrl.cls

public with sharing class trTestCtrl {
	public trTestCtrl() {
		
	}

  @AuraEnabled
  public static String getTest(){
    return 'yay from server';
  }

}