It's different for each browser and often different for the various versions of the same browser.  Best thing to do is google for your specific browser for the best answer.  However, here's a google search for all browsers:

http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=how+to+enable+javascript+in+all+browsers

And here's one of the better pages I found after a quick look:

http://www.trickyways.com/2009/06/how-to-enable-javascript-in-different-browsers/