Instructions
Texture Packs
Download a "Minecraft" texture pack that you like.
Load "Minecraft" and select the option "Mods and Texture Packs."
Click "Open Texture Pack Folder" and drop the download file from Step 1 into the texture folder. Enter a game and the new textures will be in place.
Uninstall the textures by deleting your "minecraft.jar" file, in the "Minecraft" installation directory.
Skins
Download a "Minecraft" skin file that you like. Save the .png file somewhere on your computer where you can easily find and access it.
Open your web browser and go to the official "Minecraft" website. Log in to your account and open your "Preferences" tab.
Click the "Choose file" bottom underneath your character display and select the .png file from step one.
Click "Upload" and your character animation should be replaced with the new skin. Load "Minecraft" and enjoy.