Ah, not so easy.
First, you need to use a spectral atmosphere, and in the "Sky fog & haze" tab, check the Godrays box. Next you'll need dark and dense spectral cloud layers and you'll need to play with cloud cover parameter in the Clouds tab. You need a lot of clouds, but you also need some "holes" in the sky. Now place the sun near those holes, and make a preview render. Don't trust the little preview, most of the times godrays don't show there. Make a preview render, like in the main view. Play with the sun's position and make a preview ren,der each time you move the sun.
Youn may need to change the aerial perspective parameter to 3, instead of default 1, I find it difficult to get Godrays with an AP of 1 (though it's possible)