If you are just getting started with customisation, check out the General Store in Neopia Central to find some basic wearable items for your pet.
There are always new Premium items available to customise your Neopet with in the Neocash Mall. You can also find all manner of Neopoint clothing, backgrounds and trinkets in shops throughout Neopia!
Wearable items show up in your inventory with a blue background and a convenient "(wearable)" message beneath them.