There should be no reason to change the port number.
If you change the port number as suggested by Elamathy and Apache now starts it just proves that something else is using port 80.
Find out what that is and then make a decision as what to do about it.
If you change the port number you will have to append that on all your address's
Like [
localhost]
ITS A MAJOR PAIN! AND TOTALLY UNNECESSARY !! unless you want to run 2 web servers at the same time, which is never a good idea unless you are a lot closer to being an expert than most of us are.