And ask the person in the shop for assistance, that's what they're there for, or search around here first for tips on how to fit a mask.
As far as fogging, lots of tips around here on that too. Pretty much any mask will fog unless cleaned and treated properly, though I find fogging is always less of a problem when snorkeling than diving. To clean a new mask scrub the lenses and the whole inside well using your fingers and toothpaste (the paste kind seems better than gel.) This removes stuff from manufacturing that will tend to make a mask fog. Rinse well. You may have to repeat occasionally. Then, use an antifog before you go snorkeling. You can buy commercial defogs but lots of things work, such as baby shampoo. Just a couple drops rubbed all over the inside of the dry lens, then rinse. (Some commercial kinds have specific instructions, like let dry first or whatever, depends on the stuff.) Lots of people just use spit and find it works as well as anything.