You want some funny Skin for your Minecraft Guy?
First of all, you have to choose your Skin. Google for "Minecraft Skins" or go to minecraftskins.com. You can get thousands of Skins!
Download your favourite Skin and go to Minecraft.net and log in with your Minecraft Account.
In the menu click on Profile, here you can choose by "browse" the skins on your Computer. Upload it and its done :-)