CentoOS Server Tune-Up

I have a dedicated cloud server with [login to view URL] (the pack is Ankola 3). Over the weekend it just slowed way down. Now the page loads take forever. It looks like my server is running under a high load and server resources are being maxed out, but I don't think we're running anything that heavy. Is there a web administrator out there who would know how to investigate and see if they can figure out what is causing my site to slow down? Is there a way to just take a peek around and see if you spot some problems or errors? I have cPanel/WHM control panel with 3 domains on this 1 server. Anyone peeking around inside needs to be very careful not to crash my sites or lose any data. I have full root access and you can use Putty to SSH to server. The server type is a 2 CPU cores, with 4.8 GHz total power, with 4 Gb RAM, 150 Gb of disk space with the Linux CentOS 6.4 operating system. I run PHP and MySQL. Thank you.