If you have a large website with lots of pages and posts then consider using a MegaMenu plugin. You can see an image below of the one I was using on this site with a previous theme.
There are literally tons of MegaMenu plugins for WordPress on the market now, as well as loads of them for other types of websites, you can see another type beautifully in action on popular websites Mashable and Buzzfeed.
So do a bit of research before trying and I would always advise buying a premium version rather than using a free one, they are not expensive and at least it means that the developer of the plugin is far more likely to keep it updated.
And have fun with it, they all appear to come with a multitude of configurations.