The following customization sets a default target1 file for notes, and defines a global key2 for capturing new stuff.
(setq org-default-notes-file (concat org-directory "/notes.org"))
(define-key global-map "\C-cc" 'org-capture)
[1] Using capture templates, you can define more fine-grained capture locations, see Capture templates.
[2] Please select your own key, C-c c is only a suggestion.