you need the tidal bell and the silver wing
You need to go to Pal Park.
The Silver Wing is what you need to be able to see Lugia. In between Olivine City and Cianwood City, there are the Whirl Islands. There are four entrances (you need HM07 Whirlpool to reach the islands) but only one of four entrances will lead you to Lugia. Other than merely having the Silver Wind in your possession/bag, you don't need to do anything else with it.
Darkrai isn't available in Soul Silver. You need to trade someone else who has it.
There is no such thing as the Shadow Luiga. To catch the Shiny Lugia though, you need to find the code.
you need surf to get to the four ilsands
You need to catch Luiga or Ho-oh before you go to Kanto. (personally I don't know why but it's in the game)
you need the silver wing and the tidal bell. you get the silver wing after you get team rocket out of the radio tower and the tidal bell from the kimono girls after you beat all of them. hope it helped ;D
You need to complete the gyms and the elite four in johto first
To catch Ho-oh you need a clear bell and the wing. For Lugia it's the silver wing and a tidel bell.
Whirlpool and if you don't like lots of randome pokemon battles a a few max repels would not hurt
You don't 'use' it. You need silver wing to get Lugia. It's located in one of the four islands to the south without Silver Wing in your pack it won't apear. Same with Ho-Oh except with the Rainbow Wing.
water, carbon dioxide, and the sun, not much else... hope that helped! (: