Snorlax is only weak against Fighting-types. Since it has a high HP and Sp. Defense stat, multiple hits may be required or with a powerful move like Close Combat. Sp. Defense will knock off, for example, Aura Sphere, since it's a special attack.
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Snorlax is a Normal type Pokemon, so Fighting attacks would do 2X damage to it (super effective). Snorlax is also immune to Ghost type attacks, and is damaged normally by everything else.
electric and ice are effective against flying
Fighting-type moves.
Bug, ghost, and dark types are most effective against psychic types.
I think after you get the National Dex in Sunnyshore City you can Surf to the 'Pokemon Statue' and press 'A' against it and Snorlax will appear.