If you want to design GUIs, try "sudo apt-get install glade". This lets you make interfaces for GNOME, and then export them and code them in the language of your choice.
Good, simple text editors are:
Gedit- Gnome, try installing the plugins or extras from synaptic
Leafpad- Really simple
Nano- Good commandline
Vim- same as above
Emacs- same as above
Of course, you can use any text program for coding.