Use Firebird site to download the correct "server package" for your system. First, select the version of Firebird that you would like to install. Next, select the appropriated installer for your system. Example, for almost any version of Windows 32 bits, you would select under 32-bit kits the option with "Windows executable installer recommended for first-time users".
Execute the installer and follow instructions. For first-time users, you probably will not need to change any configuration on installer.