If you use more than one email address to sign in to different services from Microsoft, you'll want to choose one to be your "primary" Microsoft account going forward.
A single Microsoft account—one email address and password—is a simple and easy-to-remember way to sign in to all of your Microsoft experiences. This includes PCs and tablets running Windows 8 or Windows RT, Windows Phones, Xbox 360 consoles, and services like Xbox LIVE, SkyDrive, Outlook.com, and Bing. You can use the same Microsoft account to check your subscriptions to services like Xbox LIVE, buy apps in the Windows Store and Windows Phone marketplace, and manage your payment info.
This depends on which email program and other services you're currently using, and what info associated with that account you'd like to preserve or change. Some accounts are harder to change than others, and once you sign in, it can be difficult to change your mind without the risk of losing info or purchases.
For example, if you sign in to Outlook.com and SkyDrive with one Microsoft account, but use a different Microsoft account to sign in to Xbox LIVE, it's a good idea to choose the email and password you use for Outlook.com and SkyDrive as your primary Microsoft account. Then, change the email address associated with your Xbox LIVE gamertag, and your gamerscore, avatar, and all of your achievements and other info will be available on all the devices you sign in to with the primary Microsoft account. To learn how to update your email address for Xbox LIVE, visit Xbox support online.