GiNaC is packaged by a couple of software distributors, so you may want to try a conveniently packaged binary first (but please don't send us bug-reports about these packages since they may be outdated). We are currently aware of packages in Debian (status), SuSE, Fedora, and FreeBSD. If you are stuck with Windows, Alexei "varg" Sheplyakov provides ready Windows builds. There is also the Knoppix/Math project which includes GiNaC and lots of other interesting software.
Before manually installing GiNaC you should have downloaded the library CLN and installed it on your system. GiNaC is worthless without that library. You can get it from the official site https://www.ginac.de/CLN/ or maybe even bundled with your favorite software distribution (e.g. Debian).
After that, you may download ginac-1.8.1.tar.bz2. Alternatively, you may directly pull from our official git repository from https://www.ginac.de/ginac.git/ or git://www.ginac.de/ginac.git.
Please read the file INSTALL for installation instructions. Good luck!