Not Much to go through in Johto at the moment so go talk to Professor Elm in New Bark Town and get the SS Anne Ticket. Then head to Olivine City and enter the SS Aqua through the passage that was previously blocked by a guy.

S.S. Aqua

Now that you're on the SS Aqua you can go around battling the gym trainers aboard. To get off you first have to find a man's granddaughter who is with the captain of the ship.
To get there you must find a lazy Sailor who is in the room next to yours. Battle him so you can get past another Sailor blocking your way later in the ship. Find the girl, and you'll be sent back to the Grandpa's cabin. He'll give you Metal Coat for your troubles. Metal Coat increases the strength of Steel attacks and evolves Onix and Scyther.
Once you have done this the captain will announce that you've arrived at Vermillion City

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