You need to get the time Flute (i think that's what it's called). To get that go to lavender town and go to Pokemon tower (again i think that's the right name but you'll know what it is when you see it) and beat team rocket. After that this old man tells you to go to his house so go to it and he gives you the time flute. Once you have the time flute go to snorlax and use it. Then you will battle snorlax and hopefully catch it.
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if you are talking about the snorlax asleep on the road... you will first need to obtain the poke flute (i cannot remember how to get it sorry) play the flute and snorlax will wake up
Unfortunately there are no wild Snorlax in Pokemon Ruby, Sapphire, or Emerald. The Snorlax must be traded from Firered or Leafgreen, or gained using a gameshark.
Lavender town is in Pokemon red, blue, yellow, firered leafgreen. NOT GOLD. also the tower in LAVENDER IS POKEMON TOWER. Defeat it to get pokeflute to get past snorlax
Get the flute
Yawn is not taught by a move tutor it can be learned by some Pokemon such as Snorlax also Slakoth and Slaking.