Introduction to Content Types

http://msdn.microsoft.com/en-us/library/ms472236.aspx
Create a site content type based on the Document parent content type

On the Site Actions menu click Site Settings and Modify All Site Settings.
On the Site Settings page under Galleries, click Site content types.
On the Site Content Type Gallery page, click Create.
On the New Site Content Type page, type a Name for the content type (for example Samples), select Document Content Types from the Select parent content type from drop-down list box.
Select Document from the Parent Content Type drop-down list box, select Custom Content Types from the Existing group drop-down list box, and click OK.
On the Site Content Type page under Settings, click Advanced settings.
On the Site Content type, Advanced Settings page, select Upload a new document template, and then click Browse to browse to and select the excel document template created in Step 1 to assign to the site content type.
On the Site Content type Advanced Settings page, click OK.

Add the content type to a document library created

Navigate to the document library to which you want to add the content type.
On the Settings menu, click Document Library Settings.
On the Customize page under General Settings, click Advanced settings.
On the Document Library Advanced Settings page, select Yes for Allow management of content types?, and click OK.
Back on the Customize page under Content Types, click Add from existing site content types.
On the Add Content Types page, select Custom Content Types from the Select site content types from drop-down list box, select the content type you created (here: Samples) from the Available Site Content Types drop-down list box, click Add to add the content type to the list of Content types to add, and then click OK.
On the Settings menu of the SharePoint document library, click Document Library Settings.
On the Customize page for the SharePoint document library under Content Types, click Change new button order and default content type.