You will probably get as many different suggested languages as you do responses
(we all have our favorites!), I'm going to have to second the Python suggestion
earlier. Python is a fantastic language, and is tremendously useful AND
powerful. An added benefit is Python and Ubuntu are closely linked (a lot of
the control programs for Ubuntu are written in Python), so this gives you a
ready source of existing programs, on your computer, to study.

There are a few good *free* tutorials on Python out there:

docs.python.org/release/2.6.6/tutorial/index.html
http://www.ibiblio.org/g2swap/byteofpython/read/
http://greenteapress.com/thinkpython/html/

These should be more than enough to get you started, and if you want to go
further with Python, there are lots of good books out there