Organic Groups - define content type to be of Group type
On the create content type page you should have a vertical tab named Organic Groups. If this tab is missing, make sure you have enabled the Organic groups UI module. Clicking the Organic Groups tab reveals a couple of check boxes. Check the one for Group and hit save to make this content type a group which you can associate content with.

The other check box Group content defines that the content type can be posted into a group, i.e associated with the group you previously created.

To get you started you can also enable the OG example module.

The above is mainly for OG 7.2.