Important decision when creating a managed package is the namespace this is a prefix applied to all your components (custom objects, visualforce pages, lightning components etc…)
The namespace prefix is an important part of the branding of the application since it is implicitly attached to any Apex code or other components that you include in your package. It can be up to 15 characters, its recommend that you keep it to less than this, as it becomes hard to remember and leads to frustrating typos if you make it too complicated. Avoid underscore characters. It is a good idea to have a naming convention if you are likely to create more managed packages in the future (in different packaging orgs).
The following is the format of an example naming convention:
[company acronym – 1 to 4 characters][package prefix 1 to 4 characters]
When the namespace has not been set, the Packages page (accessed under the Setup menu under the Create submenu) indicates that only unmanaged packages can be created. Click on the Edit button to begin a small wizard to enter your desired namespace. This can only be done once and must be globally unique (meaning it cannot be set in any other org), much like a website domain name.