To reset the password for a Mojang account (you log in with your email address) visit account.mojang.com/password
To reset the password for an older Minecraft account (you log in with your username), visit minecraft.net/resetpassword to reset your password.
If you have attempted to reset your password and are not receiving the reset email, please visit our article on not receiving system emails. Keep in mind that it is possible to have multiple accounts or account types. Minecraft accounts are explained here.
If you are unable to reset your password, please locate your transaction ID, then contact customer support.