Given that you are unfortunate enough not to live near Carrollton, Texas, be sure to buy your dive mask from a local dive shop. Try on several to ensure you find one that fits. To determine fit (please excuse me if I'm stating the obvious), place the mask on your face without the strap on your head. Inhale through your nose and tilt your head forward so that you are looking at the ground. If the mask stays in place, you've got a good fit.
You will choose between two styles: two-lenses and one large lens. The choice is a personal one.
You will see masks with and without purge valves under the nose. I recommend without the purge valve for a couple reasons. One, it's one less failure point. Two, as a snorkler, if you have a mask flood, you simply lift your face above the surface.
You may be tempted to get a mask from a store like Walmart. Resist the temptation. The masks at your LDS will be top quality. You may find used masks that the LDS used in their training program. I found an expensive Cresa mask for my wife in the used bargain bin at ST for $10.
Some masks are framed; others are frameless. As a snorkler, there is little difference. Many divers fold a frameless mask and store it in their BC as a spare.
Mask strap: Get
one of these. First, it advertises our favorite LDS. We want them to stay in business. Second, the mask is easier to don/doff. Silicon straps will pull your hair. Now, if you happen to be bald, go for silicon. Every bald diver I've seen uses silicon straps because they grip. I could be wrong.
Snorkeling vest: an excellent piece of safety gear. If you get tired, inflate. If you're swimming on the surface and don't plan to dive down, inflate and cruise face-down.
Fins: for warm water, go with full-foot fins. I use the strap version of the
Aeris Velocity for my diving. They are very powerful with little effort.