On your Dashboard under Pages - where you create a page - if you look at the right hand side rectangular area there is an option that asks you whether it is a main page(parent) or under another page - check the drop down option. You can select under which tab you would like your sub menu to come user.
For example Main Menu1 is your Initial page. Under this you need Sub Menu 1 and Sub menu 2. Create Main Menu1 page. Then create separately Sub Menu1 and Sub Menu 2 and place it under the Main Menu1 page as its parent. You can even change the order in which you want it to appear. Save and publish. You should be good to go!