Well, just like all the other gyms, you fight your way through! but to get in, go to lake rage first, and complete the sequence of events happening after you catch red gyarados. Then you may enter, and defeat the gym, (best if you have a fire type because that gym is ice type).
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Mahogany town.
You can find TM Thief in Mahogany Town in the Rocket Hideout
In Johto: New Bark Town, Azalea Town, and Mahogany Town. In Kanto: Pallet Town.
You can't get to it in soulsilver.
you definitely need help here, so go read everything on bulbapedia and serebii and psypokes, then here is the solution... mahogany town is in mahogany town!