Your mask should fit well and be comfortable. Most masks fit all faces, but you should test them (especially if renting) to make sure it is the right one for you. There are clear or black skirts (the silicone part that sits on your face), different shapes of lenses to allow for maximum viewing, different nose shapes, as well as different volumes.
Test a mask by holding it against your face (without the strap on) and inhaling through your nose. Be sure all hair is out of the way, including facial hair. If the mask sticks, or creates a suction, and you need to pull it off of your face, you should be fine.
If your mask is brand new you will need to remove the special film on the glass. This silicone preservative film is on all brand new masks and will constantly fog if it is not removed. Either scrub gently with toothpaste (the white kind) or burning it off with a lighter. Using white toothpaste (not gel) rub all inside glass surfaces well. Let it sit overnight and then rub again and rinse in the morning. A lighter is a little more dangerous so ask someone who has done it before successfully to do it for you or show you how. If you do not remove the film, your mask will fog no matter what else you try.
Defog a mask before a dive or snorkel by spitting into the inside of the mask and rubbing it all over the glass. Finish with a very quick rinse with either fresh or salt water. Do not worry about droplets, they will disappear once you are in the water. If you feel uncomfortable using your own spit, you can use fake spit sold in a bottle at a dive shop, otherwise known as mask defog, or some people swear by baby shampoo.
Do not tighten your mask strap too much. This is a common mistake that actually increases the likeliness of a leaky mask due to the fact it makes more wrinkles in your face. Your mask strap should just be tight enough to hold the mask on your face. Once in the water, the pressure will keep the mask on your face even if the strap is not around your head!
Make sure there is no hair or mask strap in the way of the mask skirt. This will lead to leaking. Take your fingers and trace all the way around your mask to ensure there are no invaders. Men with a mustache may have problems with leaking under the nose. Experienced male divers choose several methods to save the mustache and still have a good mask seal including shaving just a strip below the nose and/or using a little vasoline, or silicone gel, to help create a seal.
If you get water in your mask you can try to exhale it without removing the mask completely. This is a skill learned and mastered in the Open Water Diver Course. Simply breathe in through your mouth, exhale through your nose (think blowing your nose) while you press on the top of your mask and look up. The water should exit the bottom and sides easily. A little practice will make a big difference.