To use apt you'll have to do the following procedure:
become root user
install the apt package with either rpm -ihv or rpm -Uhv. This package can be obtained from your vendor's site (if he supports apt or maybe even from your distribution CD).
run apt-get update to download the package's database from the default server
run apt-get check, so that apt can verify if your box is OK and all package's dependencies are correctly satisfied
Note: If your system isn't seriously broken, you'll be able to fix it with apt-get -f install. If its broken, you'll have to fix these dependencies by hand (I mean, using the rpm command) before start using apt.
If everything goes fine, you'll be able to use apt-get's and other apt's commands listed later in this document.
You'll see how to change this server for others later on this document