This is how you catch Ho-oh:Get the rainbow wing from a man in Pewter City, then Ho-oh will be waiting for you at Bell Tower. He will be level seventy.
Mew is obtainable by nintendo events only; these events generally give you special or rare pokemon you simply can't obtain outside of these events, and in the case they give you a pokemon you could obtain in one of the games, it'l have a special move, ability. etc. They're often distributed at stores like Toys R Us, Gamestop/EB Games, and other toy stores/locations. Sometimes they'l put less rare pokemon over wi-fi too, although some of the rare ones will show up on wi-fi occasionally.
Rayquaza isn't obtainable in Black or White. Ruby, Sapphire, Emerald, and Heart Gold/Soul Silver only. In HG/SS, you'll need to get a Groudon caught in Soul Silver, a Kyogre caught Heart Gold, and bring 'em both to the Embedded Tower, or whatever it's called.
If you ever need to know where/how to obtain any pokemon, serebii has all your needs. Just use the correct Pokedex for whatever generation of games you need. DPtHGSS is for Diamond, Pearl, Heart Gold and etc., RSEFRLG is for Emerald, Fire Red, Pokemon Colosseum and Pokemon XD, etc. So on and so forth.