You just go to the store and buy clothes. Having trouble finding a store in Lumiose City? Then the simplest solution is to go to a taxi driver and talk to him...
Lol I tried that just now and the taxi driver tried to get the money, found out I don't have enough and then he fought me in a Pokemon battle (he tried to unleash a Lv66 Krokonok on me that I cleaved In two with Aegislash).
Then is said that his gas tank in his tax is empty, so I just got out...