Zapdos-You need the hm surf and a Pokemon who can use it. Go to the entrance of rock tunnel and there should be a patch of grass above it. There is a gap in the fence where there is water. Use surf and feep going until you get to land. Go into the POWER PLANT and find your way through. At the end fight Zapdos (lv 50) and catch it! Try saving before you battle it so if you make it feint turn the game off and then on again.
Articuno-You need a Pokemon who knows surf and a Pokemon who knows strengh. Go to Fuchsia city. Go to the bottom part of the city and head down. Surf across water until you get to land. Go into a cave which is seafoam islands. Find your way through and you will find Articuno! (lv 50) battle it and catch it! Save before battling.
Moltres-Once you defeat the cinnabar island gym a guy called bill will take you to island one.You need a Pokemon who knows surf and strengh. On island one go upwards and you will find a body of water. Use surf until you get to Mt ember. Find your way through it and at the very top you will find Moltres (lv 50.) Battle and catch it! Save before battling.
Mewtwo-After you defeat the elite four and found the ruby and the sapphire go to Curulean city. Head up the bridge and turn left. Go down through a patch of grass and use surf. Then you will find a cave entrance which is Curulean cave. Find your way through it-it's quite hard- and at the end you will find mewtwo (lv 70) Only catch Mewtwo in a ultra ball or a master ball. Save before catching it.
You can get other ledgandaries after you get the national pokedex from Prof. Oak.
you can find the following
zapdos
moltres
articuno
mewtwo
either raikouu, entei or suicune depending on what your starter is.
charizard- suicune
blastiose- raikou
venusaur- entei
all except unow and all legendaries
Zapdos, Articuno, Moltres, Raikou/Suicune/Entei (depends of which starter Pokemon you choose) and Mewtwo and Deoxys (event only) and Luigia and Ho-oh (also event only).
catch all 150 original Pokemon i believe if not its you have to catch all of them from the hoen region too
yes you have to defeat all the champions before catching a legendary pokemon
you can't catch more pokemon. you have to get rid of some pokemon.
ALL POKEMON except legendaries and pre evolved forms.
all except unow and all legendaries
You can breed all Pokemon except: Legendaries, Babies and Eggs.
All except baby Pokemon and legendaries the exception being manaphy.
Catch all the Pokemon!
go to this page http://www.serebii.net/diamondpearl/legendaries.shtml this page has all the legendaries you can normally catch in in Pokemon diamond/pearl all other legendaries you have to transfer from a 3rd generation game
You have to trade and catch as much Pokemon as you can
Zapdos, Articuno, Moltres, Raikou/Suicune/Entei (depends of which starter Pokemon you choose) and Mewtwo and Deoxys (event only) and Luigia and Ho-oh (also event only).
g;r;g;g
catch all 150 original Pokemon i believe if not its you have to catch all of them from the hoen region too
I put a link to guide on all the legendary pokemon in Pokemon Black in the related links.
You could try to catch other legendaries, breed Pokemon, look for Pokemon you want in the GTS...There isn't much to do after you've caught all the legendaries and beaten the game, though, so be careful not to catch them all in one day.