hello i have a website www.news4people.gr and i have see that has many requests in the cache files for k2 with result the website to doesnt respond ! can i reduse the size of it?

The cache folder is where images are stored.
If you want to reduce the requests, you either need a CDN solution and move the cache folder to a subdomain, or use fewer images.

can you help me with the cdn ? what to choose? and how setup it?

I cannot help you set it up but this extension is quite helpful if you do not have the required knowledge for a custom setup.

extensions.joomla.org/extensions/extension/core-enhancements/performance/cdn-for-joomla

As for CDN's cloudflare is good and it's a really reliable for a free solution. Fastly www.fastly.com/ is really reliable & fast too.

I 've used MaxCDN www.maxcdn.com/ alongside with the extension I sent you for a US based site.

Finally make sure to setup the images folder as a subdomain (multiple subdomains if possible) because this will help in your case.