Solve the puzzles.
Fantina is in the contest hall. Go there and talk to her. Then go back to the gym and she'll be there!
Just have to follow the link I gave u.
Your question doesn't specify a game and the categories were confusing so I'll assume you're in Ruby/Sapphire/Emerald? In which case you're in Sootopolis, and you have to step on each tile of ice in...
it depends............................WHICH GAME??!!??!!