You need to customize default-frame-alist in your ~/.emacs init file.
Here's an example from mine:
(add-to-list 'default-frame-alist
'(font . "-outline-Courier New-normal-r-normal-normal-15-112-96-96-c-90-iso8859-1"))
See the node "Creating Frames" in the Emacs manual for more
information about this.