You can always do it now. First, get something to back your system up. When making such a drastic change a backup will insure you have a way back in case things go south. I use Macrium Reflect for my OS backups, they have a free version that works well. AOMEI Backupper also has a free version too. You can partition a single hard drive so that it has more than one drive letter. In fact it's almost a certainty that your drive has at least 2 partitions already, they are just hidden by the system so you won't screw them up. Most Windows systems have at least a hidden boot partition and then the OS (C: ) drive. Most new systems have a few more, a recovery partition and a few others, all hidden. It's then simple to just keep your data on the second part of the drive.

Then download MiniTool Partition Wizard Free. It can be used to carve up a second partition for your data. What could be tricky is if you have too much stuff on the C: drive to make a decent Data drive. If you have scads of room your OK, if it's fairly full you will have to move your data to a USB drive and then repartition and move it back afterwards.

When you back up the system later with a backup program just pay attention to only back up the hidden partition(s) and the OS drive. Then use some sync tool like SyncBack Free and periodically sync up your data on teh second drive to a USB drive. You CAN always use something like Macrium Reflect, for both the OS and the DATA, it has a differential backup that just backs up changed data. However tools like this stick the entire backup into one huge backup file, which complicates recovery just a tiny bit. It's not hard but you have to mount the backup image to get any files out of it. When synching your files are just right there on the USB drive.