There is only one physical hard drive, so you have two primary options:
1.) Just store data to D: as you would C:. Most programs let you choose an alternative installation location during setup. Less convenient in the long run, but requires no additional setup now.
2.) Use an application such as GParted (freeware) to merge the two partitions into one, eliminating D: and transferring its storage capacity to C:. Requires a little more work not, but prevents potential issues down the road.

Hope this helps,
John