Once you have created fields for your content type, given them weights and put them in groups, and set up the display options, you're ready to create new content. Go to node/add/story (or select 'Create content' in your navigation, then choose the content type you added your fields to), and you will see an edit form that contains your new fields. Test that the fields behave as you intended by filling them out and saving the form.

That's it, you've now created custom fields in your content type! Go back and try the same thing with other core CCK fields and widgets, then try adding other CCK field modules to the mix.