It’s a broad answer because it’s a broad question. I scanned you at gtmetrix.com and it indicated you’re not using browser caching or gzip. We have some docs to help you with that: http://wiki.dreamhost.com/Htaccess_tricks#GZIP
Also you have a high number of plugins that you may not really want to use. wp-db-backup, while a good plugin, can slow your site down because you’re asking WordPress to back itself up. A cron job to run backups may be better. Also wp-statistics may not be helping. Bascially you’re going to need to remove all non-essential plugins and see if that speeds things up.
BTW, you have a lot of unused themes. You should remove them for security.