Well..you got to create a partition for Windows (how much would you like? 100GB?) for that there are some tools you can use, but I like to do that on the Windows 8 installation.
Before you start it, I recommend you to make a backup of your files, just in case.
If you don't know how to do it:
1)Insert the CD/USB with the Windows installation files
2)Restart the computer (you might need to press f12 if it doesn't boot from cd and select it manually)
3)Follow the instructions and choose to administrate partitions
4)Create a partition where your ubuntu is installed (if you dint know which one is, it should be the bigger one ;) )
5) Choose the size of the partition you want
6) There must now be a new partition called Unallocated Space. Thats where you are going to put it.
7) select it and click next
8) there you go! :D (I recommend you to use Custom installation if you don't know what are you going to do)
If something bad happens, don't worry. You can always try to install it again.