If you're in a hurry, the Google team suggests concentrating on your ABCs:
Acquisition
This metric looks at where your website traffic came from. It could be a ‘referral’ – a link from another website to yours. Or it could be ‘direct’, suggesting somebody typed in your URL or clicked on a bookmarked link. Then there’s organic search, social media and many more!
Behaviour
It’s one thing to know how users got to your site. It’s another to know what they did when they got there. Behaviour metrics look at things like landing pages, exit pages and time spent on page. If you want to get fancy, you can even view ‘behaviour flows’ to see how users moved through the site.
Conversion
This is most people’s most important metric. While a conversion could be a simple e-commerce sale, it’s also anything you consider to be a goal for the website. For example, you might want to measure PDF downloads, video views or leads from contact forms.