Pokémon SoulSilver Version Tips

Getting to Lugia in the Whirl Islands.
To catch Lugia, first, defeat Pryce, the Seventh Gym Leader. Now you must defeat Team Rocket after they take over Goldenrod City and the Radio Tower after getting a call from Professor Elm. Defeating Team Rocket will award you the Silver Wing, a Key Item needed to get Lugia, and cause Team Rocket to disband entirely.
Next, defeat the 8th Gym Leader Clair, of Blackthorn City and win the Rising Badge from her.
Professor Elm should then give you a call afterward and tell you to swing by his lab back in New Bark Town.
He'll hand over a Master Ball, and tell you the Kimono Girls were looking for you, and are awaiting your arrival in Ecruteak City's Dance Hall. You must defeat all Five Kimono Girls consecutively with no breaks in-between. After you defeat them, they'll hand over the Tidal Bell, another key item to gain access to the Whirl Islands, and to Lugia.

Lastly, fly to Cianwood City and bring a Pokemon that knows Whirlpool and Surf. A few Max Repels and Escape Ropes wouldn't hurt either. The Kimono Girls are awaiting your arrival at the Whirl Islands, so enter the Top Right Hand Island. This will lead you down to Lugia. The Island is pretty straight-forward, so you shouldn't worry about a Pokemon knowing Flash. Lugia will be at the bottom of the Whirl Islands at Level 45, so be prepared.