Snom is a new Pokémon in Sword and Shield. They can evolve into Frosmoth, although some work is involved. First, you'll need to find Snom.
Snom is a rare spawn and can be found on Steamdrift Way on Route 8, or on Route 10. On Route 8, it will spawn in overworlds, so this is by far the easiest place to find it. On Route 10, it will be a hidden encounter, so you'll have to look for exclamation marks to try and find it.
Snom's evolution is unusual because the Pokémon's level doesn't matter. Instead, Snom must be happy to evolve. You will need Snom to hit 220 Happiness to evolve into Frosmoth. To increase a Pokémon's Happiness points, you'll need to have it in your party and win battles with it. Playing with Snom in the Pokémon camp will also help, as will feeding him lots of curry.
As soon as Snom reaches 220 Happiness, all you have to do is wait until late at night, then level up the Pokémon once and it will evolve into Frosmoth.
If you need more help with Pokémon Sword and Shield, we have a number of guides you might find useful. From how to use PC Box Link, how to change the nature of a Pokemon, and how to trade Pokémon. If you're looking for some of the rarer Pokémon, we can show you where to find Dreepy and Ditto too.