There are easy to follow GTK+ 2.0 tutorials. I am learning by referring to the official gnome gtk+ tutorials and from

When compiling most often many of us newbies have got the error: "No such file or directory" even if you have all the gnome-gtk-devel and other required packages. Also you don't need to set any environment variables to make it work.

gcc: pkg-config --cflags --libs  gtk+-2.0: No such file or directory
simple.c:1: fatal error: gtk/gtk.h: No such file or directory
compilation terminated.

If that's the case then just see what command you have entered. Most probably you might have entered:

gcc -Wall -g simple.c -o simple 'pkg-config --cflags --libs  gtk+-2.0'
Note that the quote in which the pkg-config is written is ' character which is near the Enter key. If so then that's the mistake. The character must be ` which is in the tilde (~) key, above the Tab key.

So the correct way will be:

gcc -Wall -g simple.c -o simple `pkg-config --cflags --libs  gtk+-2.0`
The character ` is called backtick. We can also use $ instead of backtick.
gcc -Wall -g simple.c -o simple $(pkg-config --cflags --libs  gtk+-2.0)

It does the same thing, outputs the list of files, to be included and linked and likewise. To know more about that you can read the docs.