A few questions after my first play

[stanfc]

I have been sick for awhile and have been having memory problems. I can't seem to to find how to attack another unit except with melee. What keys or other thing do I need to have my units fire against other enemy units? I looked in the getting started and help files and did not see how to fire my units against another unit.  Any help would be appreciated losing your memory is a very tough problem.  Thanks