While there are tools to do this, be sure you backup the drive to some image file first so you can recover with ease.

With that out of the way I like the following.

1. Be brutal and uninstall all apps you haven't used in 1 year. 6 months. 3 months?
2. Try CCLEANER. Do not change from its defaults and only use the file cleaner.
3. OK back to hard disk work. Backup the image for free with CLONEZILLA. No tutorial from me as there are plenty out there.
4. With step 3 out of the way I can boot GPARTED to shrink D, move D to the end and extend C. Again, no tutorial from me since there are many out there.

Hope this helps,