I'm trying to compile a program from source first time. I changed the working directory and entered the "./configure" command, but when it was checking for some required things, I got an error message:

checking for gtk+-2.0 gdk-2.0... Package gtk+-2.0 was not found in the pkg-config search path. Perhaps you should add the directory containing `gtk+-2.0.pc' to the PKG_CONFIG_PATH environment variable No package 'gtk+-2.0' found
configure: error: Library requirements (gtk+-2.0 gdk-2.0) not met; consider adjusting the PKG_CONFIG_PATH environment variable if your libraries are in a nonstandard prefix so pkg-config can find them.

I can't understand much of this, so could anyone tell me, what exactly to do and how?

I'm running on Ubuntu 5.10 with Gnome.

You have to install gtk2.

It's already installed.

What about these PKG_CONFIG_PATH's, can anyone tell me what to do with them?

You need to install the development packages for gtk+-2.0. Usually called something like gtk-2-devel