To change clothes, gender, or anything else about your in-world ‘avatar’, simply login and click on the ‘Customize’ icon along the top of your page.
The menu will open from there, and you will be able to change anything you like about the appearance of your character.
Aside from Custom Textures, there are dozens of built-in clothes and accessories you can choose from to adorn your avatar. These will be located in the Customize menu. Make sure to click ‘Show Changes’ when you are done, or else your alterations will not appear.