I'm going to make some guesses here.
There is one physical disk in your machine.
You want a C: drive and a D: drive for some reason.
You do not have a restorable backup of your entire system.
Please correct these if any are wrong.
What version of operating system are you running?
If you need to have some of the space on your single hard drive dedicated to the OS as C:, and want to have some of the space allocated to the D: drive, this can be done. It involves a small risk of losing everything on your computer, which is why you should start with an external HDD and making an image backup of your machine to it. Should the machine get corrupted, you then restore from the external drive and everything runs again.
What you need to do is shrink the C: partition, having the free space squeezed out "to the right." Depending on your OS version, you may be able to do this in the Disk Manager, or you may need third-party software like EASEUS Partition Master (free). Once there is free space to the right of your C: partition, you make a partition and format it.
Specific details will depend on answers to
o As I asked above, what version of the OS do you have?
o How big is your drive? How much free space is left?
o Can your laptop boot from an optical drive, in case we have to use third-party tools?
EDIT: Hawkeye, we crossposted and you said it so much more succinctly than I did.