For this you need to buy a costume at one of the stores (it autoequips).
The cheapest for Female character are 10.000coins (Jasmine & Tinkerbell's friends costumes) i would think Male will be same but cant confirm.
You can see stores on the park map (
) then Park map and select the area you are currently in and go to nearest one to you.
Note: Didnt unlock for me till i returned to the park.
EDIT: KACG SoMM pointed out in commented cheapest Male costume is also 10k coins