Advice about problems in Webpagetest? (newbie)

In Webpagetest for my page www.net-lab.gr the main problems are:
-First Byte Time (503 ms)
-Use a CDN for all static assets
-Leverage browser caching of static assets
-Combine static CSS and JS files

Could you please advise me how I could correct the problems above or some of them?

Thank you!