First of all you will have to install 'scons'. See scons user guide on how to get started.
Mapnik is relying on a number of boost libraries (filesystem, python, regex, spirit etc). You can download the latest release (1.32.0) or grab a CVS snapshot from http://boost.org. You don't need to build anything, just unzip the source into read/write location on your disk.
The same requirements apply for AGG library (available from http://www.antigrain.com)
This should compile, link and install all relevant libraries/headers in PREFIX.
By default release versions are built, but you can specify "debug=yes" to build debug versions. At this time only GCC toolkit is supported and you'll have to modify SConstruct/SConscript files if you're planning to use something else.