Instructions
Head south out of Celadon City onto Route 16 and the Cycling Road. Route 16 is the first location in the game where you can capture a Slugma. Route 16, 17 and 18 all connect to one another, and you can capture a Slugma on all three routes.
Wander into the tall grass on Route 16, 17 or 18. Pokemon encounters occur when you enter the tall grasses along these routes. Slugma are rare -- only five percent of all the wild Pokemon you encounter on these routes will be a Slugma. They can appear at any time of day or night.
Walk in the tall grass until you encounter a wild Slugma. You will automatically be engaged in battle with it. Whittle down its health. Since Slugma is a Fire type, Water and Rock based attacks are the most effective. Once you have put Slugma's HP into the red, throw a Pokeball to capture it.