In Webpagetest for my page 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?
Are you on shared hosting or on a dedicated server? Improving the first byte time is probably going to require re-configuring the server unless there is something obvious in the back-end code.
I’d actually recommend looking at your js to see if you can move it out of the head and down to the bottom of the page (or load it asynchronously at the top). This is difficult if you have inline scripts that depend on it but you can get close to a second improvement by getting the javascript execution out of the critical path.
Next up I’d take a look at the rokbox theme icons and get them combined into a sprite (and possibly merge as many sprites together as possible).
After you get those done then a CDN MIGHT make sense if you have users from around the world. If your users are all located in one region it is probably not worth it (assuming you server is in the same region as the users).
I’d stay away from the first byte times for now. The js and image spriting are going to require some development to the site template and code - any chance you have a development background? If not it could be pretty risky to make some of the changes as they will break the site.