Uploading a player skin (Minecraft for PC)
To change your Minecraft skin, sign into your Minecraft profile at minecraft.net/profile. There you can download the classic or slim-armed reference skin, edit it, and upload it for use in-game. There are also many websites that store and distribute popular skins, or if you have an iOS device, you can use Minecraft Skin Studio.
Outer skin layer
As of Minecraft version 1.8, skins can have a second layer on each body part. For information on creating a custom skin, visit the Minecraft Wiki.
Only the inner part of a dual-layer skin will be visible if you use it while playing a Minecraft edition prior to version 1.8.
If you've not uploaded a custom skin, you'll have either the classic "Steve" skin, or the newer "Alex" skin, which features slimmer arms. You can choose between the two model types, download the default models, or download the templates all at Minecraft.net/profile. Your default is randomly based on your account ID, is shown when you don't have a connection to the skin servers, and cannot be changed.
Slim-armed skins are only fully supported in version 1.8 and higher; if you have a slim-armed skin selected in a version prior, you will see black pixels on the arms.
Why does the website say that I have to pay to change my skin?
This message is displayed if you are not currently logged into an account which contains a paid copy of the PC version of Minecraft.
Why don't I see my skin in multiplayer?
If you are unable to change your skin, or do not see your skin in multiplayer, visit help.mojang.com to see if the skins server is temporarily down. If you see the default skin in-game, first check server status, then try logging out and back in to refresh. If everyone's skins show as the default, the server may be running in offline mode.
I uploaded a new skin; why did my skin not change?
Skin changes in Minecraft version 1.7.9 and later should happen immediately. If you are playing version 1.7.8 or an earlier version, skin changes may take up to an hour to be applied. If you are playing version 1.3 or earlier, skin changes will not be reflected in-game.
Why are there black lines on my skin's arms?
You'll see black lines on your arms in versions 1.8 and later if your profile has a classic model selected, but you've uploaded a slim-armed skin. To fix this, visit Minecraft.net/profile and upload the correct skin type for the model you've selected.
You'll also see this issue if you've uploaded a slim-armed skin prior to them being implemented in the game (versions 1.7.9 or earlier). To fix this, visit Minecraft.net/profile and upload a classic skin.
Skins for other Minecraft editions
Skin packs are available for purchase on the Xbox and Playstation editions of Minecraft. More information on changing skins in the console editions can be found at the Minecraft Wiki.
Skin changes are not currently a feature in Minecraft: Pocket Edition, though this is planned for the future. Unless otherwise noted, the various apps on the app store exist to easily change the skin on your PC account, not Pocket Edition.