[SalesForce] How to query Related Opportunities related List in Account

Here are some resources that will help you:

• https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_qs_HelloWorld.htm
• https://www.youtube.com/watch?v=1cO3O4yMv7w
• http://www.salesforcetutorial.com/apex-trigger-create-simple-trigger/
• http://www.sfdc99.com/2013/05/12/example-how-to-write-a-simple-apex-trigger-2
• How to write simple after update trigger on Account Object?

You can use controller extension with StandardSetController. The getSelected() method of StandardSetController returns the selected records.

Visualforce

<apex:page standardController="ts2__Application__c" recordSetVar="selectedApplications" extensions="ApplicationExtn">
  <apex:pageBlock >
    <apex:pageBlockTable value="{!selectedApplications}" var="a">
        <apex:column value="{!a.name}"/>       
    </apex:pageBlockTable>
  </apex:pageBlock>
</apex:page>

Controller Extension

public class ApplicationExtn {
    public List<ts2__Application__c> selectedApplications {get;set;}
    private ApexPages.StandardSetController controller;
    private Set<Id> appIds = new Set<Id>();

    public ApplicationExtn(ApexPages.StandardSetController controller){
        this.controller = controller;
        selectedApplications = controller.getSelected();
}

For more information, refer StandardSetController Class