First and foremost. Make a complete backup of everything on the drive. If this is your Mac's boot drive (I assume it is), it would be easiest to make a clone you can boot to. If you already have a Time Machine backup, you can use that to restore your Mac data, too. It's just much faster and easier to create a bootable clone, boot to that, repartition the main drive, and then copy the clone back. It sounds a bit complicated, but isn't hard to do.
I never use Time Machine, so this is going to assume a bootable clone.
1) If you don't already have one, purchase an external drive attach it to your Mac. Turn it on if it has a separate power switch (some models are simply on whenever they're plugged in).
2) Open Disk Utility. Select the new drive's physical drive name in the left column.
The Partition tab will be available at the left. Click that. Where the drop down menu says Current, change it to something else. If you only want one partition on the new external drive, choose 1 Partition. You need to choose something other than Current, or you can't get at the lower button (it stays grayed out). You can create as many partitions as you like. Since an external drive is likely going to be at least a 1 TB, there's no sense wasting all that space on just a clone of the internal drive. May as well create at least two partitions so you can have a bootable clone of the Mac's internal drive on one partition, and a second partition to store all kinds of other data.
You can make each partition any size you want to split up the space you have, and give each one a name that makes sense to you. For the external drive, the first partition must be Mac OS Extended (Journaled) so it will be usable to make a clone of your current startup disk. The second you can kind of do with what you want. It can either be another Mac formatted partition (same as a the first), or you could make it ExFAT and use it as a partition both Mac and Windows drives can read/write to. It can then be used on either platform to store and share files.
But you must at least duplicate your Mac's internal drive. If it already has two Mac formatted partitions with data on both, then the external must have at least two of the same type of partitions so you can back them both up.
As shown next, once you change the number or partitions and set them the way you want, the Options button can now be clicked.
If the radio button is not on GUID, change it and click OK.
Now click Apply to apply the changes to the new drive. It will only take about 20 - 30 seconds.
The new drive is now set up for your data to backup. With Disk Utility still open, click the Restore tab. If the name in the Source field is not your current startup drive, just drag and drop it from the left column into that field to change it. Drag and drop your external drive's Mac formatted partition you want to use for your bootable clone into the Destination field. Click Restore.
Do touch the drive while it is cloning the startup disk. That is, don't try working in another app or doing anything else while Disk Utility is working. It's pretty difficult for OS X to clone the drive if the contents are changing while it's copying data from one drive to the other. Just leave it be until the clone is done.
Once that clone is done, clone your second partition of data the same way, if there is one.
Quit Disk Utility.
3) Restart and hold down the Option key. When the Boot Manager appears, select the bootable partition of the external drive and press Enter, or double click the icon. The Mac will startup to your cloned drive. If you don't see two choices (the Mac's internal drive, and the cloned external drive) then something wasn't done correctly. And the only thing that could be was you missed changing the external drive's partition map to GUID.
Once booted to the external drive, make sure you are on that drive since it will look identical to the internal drive. Choose the Apple at the upper left and choose About This Mac. Check the Startup Disk name. It should be whatever name you gave the external drive's partition you cloned the internal drive to.
4) Repeat the entire process above of partitioning the drive, only to the Mac's internal drive. Then clone everything back. The only difference for partitioning the Mac's internal drive would be if you have, for example, two Mac formatted drives of data on the external drive, you will need three on the internal drive. Two Mac formatted partitions again to clone your Mac data back, and a third formatted as MS-DOS to install Windows to.
5) Restart after you're done cloning the cloned drives back. Check again under About This Mac to confirm you successfully booted to the internal drive. Dismount the external drive's icons and turn it off.
6) Boot to your Windows installer and continue. It should allow you to erase the MS-DOS formatted drive this time.
If this all sounds a bit much (it's mostly a lot of sitting around watching files copy), then get a friend or relative with proper experience with Macs to help you.
