It doesn't appear that you are under attack as your site does load. Since you're using Wordpress, you may want to consider wp-supercache in mod_rewrite mode, I think that will greatly speed up your site, and will save you some memory.
If you root access to the server login as root with putty or if you use linux ssh client (OpenSSH).
Run the commands as suggest will give you IP's and the uniq -c adds up how many of the same iP are connected,
Any IP's over 8-12 connections could be the DDOS ip's, but complex ddos attacks may only connect 2-3 times from each ip over a range of ips.
So if that fails, then have a look at htop, vnstat -l -i <network interface>, or you can use nethogs.
Monitor and try to figure it out,
Contact your provider as well as it sounds you need lots of help.
Google "Am I being DDOS" and you get a range of results that will help you.
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As I read above you are using Wordpress would recommend to install captcha to avoid registration and comment spamming. Yes Super cache plugin would help to avoid load on your server. Still not satisfied you can go for a CDN service.