We don't know how flexible and clean your framework is, how much of its code (CSS rules and HTML markup) you can rewrite and how easily you can swap fixed to fluid or relative units, etc. Then there's also the question how you'd like to make your site responsive, e.g. work yourself from the smallest screen (mobile first) up to the biggest, vice versa, optimize for tablets first... and so forth, so I really find it nearly impossible to answer your question without being vague.
Some tools are articles have been collected here: http://mobile.smashingmagazine.com/2011/07/22/responsive-web-design-techniques-tools-and-design-strategies/
Seriously though, Responsive Design is a hot topic and there are a plethora of articles, templates, guides, tutorials, and frameworks out there to choose from.
Personally, I do not recommend Twitter Bootstrap. It's very, very heavy, too bloated for my taste. There are other frameworks out there you can check, though I don't recommend any of them, either (here are some reasons):
Some framework listings:
If you do a search, you'll find plenty more resources.