http://www.tldp.org/HOWTO/html_single/Partition/
I myself still struggle with partioning and at the moment let installers do the work (auto-partitionning)the above guide is quite comprehensive and well done.
the installer will save you time and pain may you later try out manual multi-partitionning.
https://www.debian.org/releases/stable/i386/apcs01.html.en Appendix C. Partitioning for Debian
