Head to Pewter City and talk to the old man next to the mart.
He will give you the Silver Wing.
Next, head to the Whirl Islands. Bring Pokemon knowing: flash, strength, surf, and whirlpool.
Make your way to the bottom of the Whirl Islands and there you will find Lugia.
Lugia will be level 70, so make sure you bring Pokemon capable of putting up a fight.
Lowering it's HP will of course help along with putting it to sleep. Ultra Balls are your best bet unless you still have your Master Ball.
Best of Luck!