You can put it in a meta tag or robots.txt but you need to put rel="external nofollow"
If you just put nofollow PR will not pass amongst your internal pages.
Also, If you do any link exchanges you need to state this upfront with any link partners.
Most blog software is run on a template system. You would only need to change one file. Most of them call it head.html or head.tpl - look for it in the templates folder for the specific theme you are using.