There can be many things that contribute to causing a high load on your server. If you have received reports from your web host that your account is causing a high load.
First, look at the number of sites you are hosting. If the number of sites is 15< than you should consider offloading some of those sites to another web hosting account. You should try to not group database, memory, or resource intensive sites together into one web hosting account. You should spread out resource intensive sites with static or simple HTML sites. Having 5-6 MySQL intensive sites will cause spikes in your load and memory usage.
Second, you should look at all your non-static sites aka dynamically generated pages. Sites that are powered by Joomla, Drupal, WordPress, etc. You should look at the different number of plugins and modules that you have installed. If any plugins or modules are not being used then you should disable them. Having a lot of useless modules or plugins installed but not actively being used can contribute to a high load and high memory usage.
Third, using your FTP program or file manage you should look for odd scripts or rogue scripts that you no longer use. If you installed some random script months ago and it is no longer being used than you should remove it. Having rogue scripts can cause performance issues when you least expect it.
Fourth, check your cron jobs and make sure to delete any cron jobs that are no longer needed. Only have cron jobs that are needed listed.
Fifth, look at your web statistics program it can be AWStats or Google Analytics. See when you have the most visitors and compare it to when you have high loads. Does the high loads occur when you have a spike in visitors to your site? If so, you may need to upgrade to a more powerful plan.
In conclusion, the more things you have running or have enabled to run on your web hosting account will contribute to a high load. The number of sites you have hosted, number of plugins and modules will all contribute to slowing down your site’s performance when more visitors access your site. A high load can be caused by many things but badly programmed PHP scripts and bad plugins are the most obvious suspects.