Menu issues

Hi All, I’m used to conventional web design using HTML, Dreamweaver etc but really struggle with WordPress. I’m sure its really my fault but I cannot get my head around the menu system.
What I want to achieve is an entry page using Asteria theme with a slider on the “Homepage” and a top-level menu with just a couple of dropdown items each of which can access another page with entirely different top level menus.
I’m using it to document a series of holidays so the Homepage will have a toplevel menu offering dropdowns to each holiday where the toplevel menus will offer daily photo galleries, blogs(posts) and a description page.
Desperate to get this working I’ve spent hours fiddleing about and when I change the menu it changes on all pages – what am I doing wrong please?