Use the Bootcamp assistant found in the Utilities folder to partition your drive .
This creates a special and non standard partition tree with two tiny and extra hidden partitions necessary for safe double booting of the same drive.
The OSx partition is reduced however all data and the boot sequence is retained by default.
The two hidden partitions are a Boot Rom to allow EFI booting of windows in lieu of the normal BIOS Boot Rom and a second small separator both are around 160 to 200mb in size.
Once you create the Bootcamp partition you load your windows disc and follow the normal install procedure very carefully.
Caveat and recommendation make the partitions notably differing sizes as this reduces the risk of accidentally installing windows over OSx during the Windows install procedure.