Reserving Your App Name in the store ensures that no one else can publish an app with the same name. For this reason, it is helpful to reserve your app name as soon as possible, potentially even before your start developing your app. It would be really unfortunate to be prepared to submit an app with an awesome name, only to find out that someone else has already used that name! Therefore, my recommendation is to reserve your app name as soon as possible. However, regardless of when your decide to Reserve Your App Name, whether it be months before you begin development or right before your submit, here is how you do it.
Open up the Developer Portal at http://dev.windows.com/, and then click Sign In the top right
Log in with your Developer Account
Now click on Dashboard in the top left.
Choose Windows Phone Store.
Choose Submit App in the top left.
Click App Info.
Type in your new app name and click Reserve App Name
Then scroll down to fill in your App Category and Pricing. This info can be changed later on so don’t worry too much about what you choose for now.
After you have filled out that info, scroll down to the bottom and click Save.After you have filled out that info, scroll down to the bottom and click Save.
There you have it, your Windows Phone app name has been reserved!