If your device isn't running Windows 10, refer to the Windows 8 instructions.
The following steps only apply if you signed in without a Microsoft account during the initial setup of your device and have an existing account to use.
If you don't have an existing account, you can create one.
The first Microsoft account to sign in is designated as the owner of the device. Any subsequent Microsoft accounts that are used to sign in won't have owner privileges (the ability to change or modify users, etc.).
From the Windows desktop, navigate: Start > Settings icon (lower-left) > Accounts.
Utilize the touchscreen (if available) or mouse to select on-screen options.
Select Email & app accounts then select Add a Microsoft account.
Enter the email address or phone number and password then select Sign in (lower-right).
If presented with a 'Something went wrong' screen, ensure that you are connected to the internet then select Retry.
If applicable, follow remaining on-screen prompts to switch to a Microsoft account.
You may need to verify your identity by entering a confirmation code. For further assistance, contact Microsoft.