To start with, do you know how to switch between breathing through your nose, and breathing through your mouth? Try this: Put your hand over your mouth. Now breathe out into your mouth until your cheeks puff up. Now let the air out through your nose. Now puff up your cheeks again. Alternate between the two, until you can clearly feel what it is that you do to route the air to one place or the other.
Now, try standing in chest-deep water, with your snorkel at hand (no mask). Try putting your face in the water and breathing through the snorkel. Be careful to use the "mouth" setting you discovered during the first exercise.
Once you can breathe without a mask comfortably, add the mask. Now, you can (still standing in the shallow water) put your face in the water and flood the mask. Stand up, put your fingers on the top of the mask, and exhale gently through your nose. If you are getting water in the back of your throat, tilt your head a little bit so you are looking slightly down. Your mask should clear without choking you.
Once you can do this as described, you are ready to try mask clearing underwater. Remember that, if you are upright, tilting your head back will allow water to run down into your throat. You NEED to tilt your head back, if you are in a horizontal, diving position; but if you are sitting or kneeling, you do NOT. Focus on a steady, gentle stream of air into the mask. You do not need to blast the water out -- you just need to REPLACE it with air.
You can actually do almost everything I've described in a hot tub or even a bathtub, so you don't really even need a pool, and you can practice this as much as you need to at home, until you are very relaxed in doing it.