installing mssql

hi,
i really don't know how to install the ms sql. I already have a red hat linux 9.0, apache 2.0, php 4,mysql installed in another computer. however, instead of using the mysql i was instructed to use ms sql. how will i be able to install the ms sql to the computer. i've searched the net but found the installation from scratch (installation of red hat, apache, php, and mysql). since, there's no mssql on the computer I will be the one to install but i really don't know where to start and what are the specific softwares i will download.