To reset the password for a Mojang account (you log in with your email address) please visit https://account.mojang.com/resetpassword/request
If you have a non-migrated Minecraft account (you log in with your username), please visit http://www.minecraft.net/resetpassword to reset your password.
If you have attempted to reset your password and are not receiving the reset email, see this article: http://help.mojang.com/customer/portal/articles/542001
