Good morning (in europe) and welcome here!
You must navigate to the same folder where Your "C" program is.
Starting from another path is also possible, but need some more info:http://learn.clemsonlinux.org/wiki/Basic_Commands
Besides that You probably must mark it first as executable either by GUI (right mouseclick and then settings --> exetutable) or by giving this command:
sudo chmod a+x myfile
"sudo chmod 775 myfile" does btw the same!