Adjusting the Navigation Menu with CSS, where do I start?

I know about Child Themes and that there are snippets that could help, but I can’t find out how to change the navigation menu to display a Login and Logo on top, a picture below and then the navigation menu after.

Here’s the screen design I’m using for what I want to create:

View post on imgur.com

Here’s the website as well:

http://www.betaexeterpound.org.uk

I’m fairly new with WordPress and coding, so my knowledge of it is pretty much at the newbie stage.

I’ve also tried to look into the source code, but it’s a bit too complicated to understand how to find what code to edit and whatnot.