Charcter customization? how would I approach it

Well to start, you will need to have a table of all your items that you can equip, like this.

      Hats = {"ID", "ID"}
      Gear = {"ID"} --And so on.

You would also need a way to equip it, something like

      function EquipHat(num)
                Instance.new("Hat")
                Hat.MeshID = Hats[num]
                Hat.Parent = Player
      end

You might also want to check out some wiki articles on the player's character and some basic GUI tutorials. You will probably be doing a lot of table work in your project, so look into tables as well.

It's actually quite easy to create these kinds of GUIs, I hoped this made it easier for you,