Gnuplot is is a plotting program which produces professional quality graphs that may be used in scientific manuscripts. You can access this software through the PC's in Bilkent computer labs, or you can download and install it on your pc through this web page.
One of the most practical ways to learn the gnuplot software is by entering "help" within the program.
Here are a few examples to help you in starting to practice.
Here is an example of the use of the curve fitting utility of gnuplot.
Here are also a list of web pages which provide useful tutorials and/or examples: