You don't. If the legendaries you're referring to are Reshiram/Zekrom, you can't get either of them until the very end of the game, after you've beaten the Elite Four. I won't ruin the plotline for you, since that part of the game is probably the best out of the whole thing, but you can only get the one that is specific for your game. (Reshiram for Black, Zekromfor White)
You can go to battle tower, vs you rival, catch legendary pokemon you can catch garratina in dimond look on youtube.
in tin tower in ecruteack city or the cave near the whirlpool
in the celestial tower
yes
at N's castle or dragonspiral tower
no
master ball
you have to beat the elite 4 89 times
if you hack your game you can encounter a level 100 ho-oh
You can go to battle tower, vs you rival, catch legendary pokemon you can catch garratina in dimond look on youtube.
Yes, just catch one and tap it over 100 times, it will appear as a legendary pokemon.
You can't. They all run away and you have to run around the region to try to catch them. In the wild grass.
You must trade with everygame,go in the trainer tower,and finally catch every legendary.
in tin tower in ecruteack city or the cave near the whirlpool
You catch Ho-oh at the top of Celestial Tower after beating the Elite 4, ring the bell and it will fly out of the sky and onto the tower. Go catch it, it is at Lv.70. Good Luck!
Burnt tower, Tin tower and whirl islands.
at the Pokemon league