speeding up my wordpress site

I would like to know how to figure out what is slowing down my site, so I know what to concentrate on fixing.

I am using DIVI as a theme; and am using several plugins to accomplish everything I need for my member website (including WP-Client and Toolset). I use a VPS through JustHost - and I know that if I had an unlimited budget I could do better - but I don't have an unlimited budget because I work for a very small not-for-profit organization with a staff of 2. My website is http://www.nmi.org

Any suggestions and tips you have would be greatly appreciated.