How do I know if my mask is fitted right?

Here's a useful test: Press the mask to your face, without strapping it on. Just by breathing in with your nose, you should be able to suck the mask onto your face and walk around without it falling off. That proves that you have a good seal around the edge.

My scuba instructor told me to spit in my mask. Is he kidding?

No! He's not kidding! Saliva works very well as a mask defogger, for reasons that have never been adequately explained. Something in the saliva will keep the mask from fogging up. Many divers swear by it, and will use nothing else. There are commercial mask defoggers available, usually sold in little eyedropper bottles. A popular recipe for mask defogger is 5 parts water to 1 part biodegradable dish washing solution. Diluted glycerine is also effective.