I'm Going to teach you how to change skins in minecraft.
First of all you need to download the skin you want.
Second go to computer and search in %appdata%.
Third Click .minecraft.
Fourth go to bin then right click minecraft and open it with winrar archiver or 7zip.
Fifth click the mob folder find the char.png (Char means Character) then find your downloaded
skin and extract it to char.png.
Then last launch minecraft and you have a skin in minecraft.
It does not work in servers because the skin is only visible to your PC.
That is too old. Here is new guide for that: 1)go to %appdata% then go to .minecraft 2) get a resource pack or texture pack I recommend faithful 32x32. 3) put the resource pack in resource pack folder inside the .minecraft folder Then open it. 4)now u see assets folder then open it then minecraft>textures>entity> then scroll down and you will see steve.png then delete the steve.png. 5) Download a minecraft skin then go back in your opened resource pack right click and select add files to archive and find the skin that you downloaded and you're done!! I used winrar in opening the resource pack and it works in keinett launcher. Oh wait u have to rename the downloaded skin to steve before putting it on the resource pack
I have a better solution that doesnt involve corrupting your game and is WAY easier for those who are still looking for a way for others to be able to see your skin, that is IF others use the same method. Check it out here!