Entei’s a roamer, so its location changes very frequently. After you’ve encountered it once, you’ll be able to check its current location using the Pokédex, but until then, encountering it is pretty luck-based. You can either trade one to your game to register it in your Pokédex, or you can use the tall grass on the border between Pallet Town and Route 1 to find it relatively easily. Lead your party with a Pokémon between level 6 and 50 (the higher the better, but it can’t exceed 50), use a repel, and then go in and out of the route, going through the grass as you do. The repel will keep away the regular Pokémon on that route, and entering/exiting the area will reroll Entei’s location. Do this enough and eventually Entei will appear on the route.