You have to launch gnuplot first (type gnuplot and press enter), and
then when you're in give the command (in your case
sin(x)). This is what you should see on the terminal:
G N U P L O T
Version 4.6 patchlevel 0 last modified 2012-03-04
Build System: Linux x86_64
Copyright (C) 1986-1993, 1998, 2004, 2007-2012
Thomas Williams, Colin Kelley and many others
gnuplot home: http://www.gnuplot.info
faq, bugs, etc: type "help FAQ"
immediate help: type "help" (plot window: hit 'h')
Terminal type set to 'wxt'
gnuplot> plot sin(x)