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 don't have an existing account to use.
The first Microsoft account to sign in is designated as the owner of the device. Any subsequent 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 Sign in with a Microsoft account instead (below the Local Account name and info).
From the 'No account' section, select Create one.
Enter the required info (e.g., Name, email address, password, etc.) then select Next.
If presented, follow the remaining prompts to complete setup.