First, you must have beaten the Pokémon League at least once.
The first wild encounter after beating the Pokémon League will always be the legendary bird, in your case, Moltres.
After this initial encounter, you will have to find Moltres roaming in the wild 10 more times—I recommend using the Location feature on the Pokedex and then flying to that area. After flying to the area it’s in, check the Pokedex again to make sure it has not moved, then go into the tall grass until it appears (it may not be the first encounter you get, and it there is a chance of it leaving after every encounter, so it’s important to check the Pokedex regularly—I can’t stress this enough).
After 10 encounters, it will retreat to Sea Spirit’s Den in Azure Bay, where it will no longer flee and you can catch it.
Source: this, plus personal experience.
