How can i prevent my lab goggles from fogging up due to wearing a mask?

Use a mask with a nose bridge and a tight fit. Use the glasses to seal the top of the mask. Wash the glasses in soapy water and allow them to air dry.

First, if your goggles are fogging because of the mask, it means your mask isn't properly fitted.  

Adjust the wire along the top of the mask to fit more snuggly around the contours of your nose and upper cheeks. If your mask does not have a wire then you should get one that does, or find a mask that doesn't leave gap along the top edge.

If your goggles are fogging due to perspiration or condensation, make sure they reach room temperature before putting them on (they shouldn't be cold to the touch).  You can also buy fog-prevention sprays on line or at some sporting goods stores.

As a quick fix you can also coat the inside lenses with  liquid soap (like from a dispenser in the rest room). Slowly drip water over the lenses until they're no longer blurred by the soap, then let them air dry (don't rub them).  The soap will leave a slight film on the lenses that'll act like the commercial anti-fog sprays.