This is what I do, other methods vary/
1) Find a skin you like via planetminecraft or another site just by googling.
2) Download the skin
3) Log into your mojang/minecraft account.
4) Go to account settings
5) There is an option somewhere (I cant see currently b/c its blocked at work) that says to change skin. You can upload your skin there.
There is another way via your launcher, but I haven't look at that.