Which programming language do you wanna use?
Which toolkit do you wanna use?
Most apps with GUI for iLiad are written in C using the GTK+ toolkit.
Maybe you should check the official GTK+ page,
http://www.gtk.org/
have a sneak at the tutorials,
http://www.gtk.org/documentation.html
visit a GTK+ forum with example code etc.,
http://www.gtkforums.com/
Every linux textbook/or documentation of your linux distribution comes with a programming section.