Namespace is 1-15 character alpha numeric Id.
Note:
Namespace should be globally unique across all the Salesforce.com organizations.
Follow the below step to set Namespace
1. Go to Setup –> App Setup –> Create –> Packages.
2. Click “Edit” button.
3. Click “Continue” button.
4. Give a Namespace Prefix and click “Review my Selections” button.
5. Click “Save” button.
Cheers