The advice I have found from purportedly experienced gamers is that first you must acquire all the badges, a total of 16 in all, then beat 'red the trainer from pallet town' and then combat and beat the elite four. Apparently, on the beach of olivine, you will find an entrance at the top left corner.
Johto: Zephyr Badge - Flying, Hive Badge - Bug, Plain Badge - Normal, Fog Badge - Ghost, Storm Badge - Fighting, Mineral Badge - Ground / Steel / Rock, Glacier Badge - Ice, Rising Badge - Dragon
Kanto: Boulder Badge - Rock/Ground, Cascade Badge - Water, Thunder Badge - Electric, Rainbow Badge - Grass, Soul Badge - Poison, Marsh Badge - Psychic, Volcano Badge - Fire, Earth Badge - Random
Trainer Red begins with a level 88 pikachu and then his Pokemon; venasour, charizard, blastoise, are all level 84, lapras at level 80 and snorlax at level 82.
The advice from one gamer in how to defeat Trainer Red was to "start with a donphan with eq. 4 venasour typlosion or rk9, for charizard any rock type like golem, then 4 blastoise an ampharos, lapras = fighting like primerape, snorlax = primerape" I hope that means more to you than it does to me!
In order to defeat the Elite Four, firstly buy as many full heals and max revives as is possible, and after acquiring six different types of Pokemon, build up their levels to exceed 60; although leveling them above 70 will be even better, and more likely to work.