On a physical Spanish keyboard the ñ is in fact on the board directly, so it is assumed that the question implies that you use a US-English keyboard physically.
For users of those and frankly typing "internationally" or not I quite advise setting the "English (US), English (intl., with AltrGr dead keys)" layout in the keyboard settings. It's a proper superset of the US-English layout and has many keys such as ñ directly available as well, as in this case via AltGr-n (where AltGr is Right-Alt if it's not labelled AltGr on your board). See the layout picture in the keyboard settings when you set it or below.
xenopeeks's advised compose method has the advantage of not needing to remember anything in the sense that with compose,
AltGr-' e is
é in much the same way as
AltGr-Shift-` n is
ñ but you'd find that e.g.
AltGr-e and
AltGr-n is much faster if you use just a few regularly --- and both those and any potential rest you can
still compose if you want to anyway.
As basically always happens with this question there'll now be 20 or so additional replies advising even on things such as copy-pasting out of a font table but I'd quite advise to not pay anything other than the layout method and/or his compose reply any mind.
Of course, also note that if the assumption of a physical US-English keyboard was wrong you'll likely just want to
set the proper layout in the keyboard settings...
