Help! Nav bar and sidebar widget problems…

Hi all, this is my first post here and I’m a complete WordPress/CSS etc novice.

Basically, here’s the problem: I’m trying to customise my cricket club’s website and badly need a bit of help because I;m out of my depth.

I tried to add a nav bar in the header and it ain’t working properly, and I can’t understand why.

Firstly, the drop down menu doesn’t line up properly when I#ve anchored it to the header (it’s fine when positioned to the page).

Secondly, a widget in the sidebar overlaps it. Annoying. I’ve tried fiddling with the z-index but don’t really know how it works. As I said I’m a complete beginner at this and learning as I go.

Can someone please help suggest (in a very simple way) what’s wrong? If you can, it’ll be much appreciated.

Cheers, here’s the site: https://www.krcsc.co.uk

(And before you say it, yes the site’s colour is horrible but I didn’t choose it!)