For Windows machines:

Download putty from
http://www.columbia.edu/acis/software/putty/
You may have to enter your uni and password to download it.

Goto Programs -> Putty -> Cunix

Enter your uni for login and then it will ask for your password.

Once you login, you will see a $ on the left of your screen.
Create a directory (make directory) called testdir by typing:
mkdir testdir
Change the directory by typing:
cd testdir

Now open a file with emacs editor
emacs sum.c

Type in your program
Save the program using Ctr-x Ctr-s
Exit from the program using Ctr-x Ctr-c

Now compile your program by typing 
gcc -o sum sum.c
And then run your program by typing
./sum