you havn't told that what is your distribution.. mostly it is installed during installation.. if redhat then do service httpd status
it it says it is not running then try to start it.. using service httpd start
if it is in your system it will start.
It is not then consider a rpm of apache that will make it easier to install.
If it is there and running then you can put your files ie. html, php in /var/www/html/ and open in your browser .. like..
http://localhost/filename.htm or on lan http://ip addredss/file.htm, currently it is confugered for only one account but you can make it to do several sites.. called virtual sites. do to so i suggest you to read some apache documantaion.. that will also help u to do more..
make sure you have also installed php support.. to run php