Linux does not use the concept of "C" and "D" drives. In Linux, there is single filesystem tree, and various drives (in fact, partitions, not whole drives) can be mounted at various points of this tree. These are the mount points you are asking about.
You need to familiarize yourself a bit with the Linux filesystem structure. It starts with root directory (not to be confused with the root user home directory, /root), which is at the very top and its name is simply /. There are various subdirectories under this directory, like bin, dev, etc, usr, var and others - all of them have specific purpose for the system and basically you shouldn't touch them manually (except sometimes editing of some configuration files in /etc or its subdirectories may be needed - as this is the directory that holds configuration files for various system components). The one that will be interesting to you is the /home directory. This is the directory where home directories of all users are located.
In Linux, the concept of home directory is very important. Each user has their own subdirectory under /home, with the same name as the username. Home directory is the place where all per-user configuration is stored (contrary to global configuration common for all users which is in /etc) and also is the place for storing user data, like documents, photos, videos etc. (in Ubuntu, you even have dedicated subdirectories for these types of data in user's home directory).
Something similar also exists in Windows, and is called "user profile directory" which is located somewhere like C:\Users\username (and also has subdirectories for "My Documents", "My Videos" etc.), but it is not so important and prominently used in Windows as it is in Linux (in earlier versions of Windows there was no concept of "user profile directory" at all and this thing didn't exist, while in Linux it is there from the very beginning).
A commonly used configuration when you have two drives like you have is to use the first drive (SSD) for the root directory (/) and all its subdirectories except /home, and use the second drive (HDD) for /home. Thus we may roughly say that the first drive will be the "system drive" and the second one the "data drive" - this is probably closest to the concept how do you use "C" and "D" drives in Windows. However, be aware that all the software that you install in a standard way, from a repository, will be installed to "system drive" - in Linux, the locations where software is installed are fixed and there's no easy way to change it. Only some third party software is "self-contained", ie. can be installed in any place, in particular within your home directory (especially the software distributed in form of so called "AppImages" - these don't require installation at all, you just put them anywhere, make executable and run). You can also usually change the installation location if you compile the software yourself from source. Other than that, the software goes to fixed places within the "system drive".
So you have to create at least one partition on your "data drive" and as many partitions on the "system drive" as the installer requires. Probably it will be three partitions: EFI partition (if you already have Windows then you probably already have this one, it can be reused), boot partition and the main root partition.
In Linux, your first drive is usually called sda and the second one sdb, so the partitions will be sda1 for EFI, sda2 for boot, sda3 for the root partition, and sdb1 for the single partition on the data drive.
Use installation mode with custom partitioning. You should choose / as the mount point for sda3, /boot as the mount point for sda2 (the installer will probably set these automatically as default), and /home for sdb1 (this will not be set automatically, you need to explicitly set this).
Note that you will be using the drive and partition names (like sda or sdb1) explicitly only under very specific circumstances, like partitioning/installation. During normal system use, you will just refer to the path to a particular directory/file. For example, /usr/bin directory (it's the directory where most Linux system commands/programs are located) will be placed on "system drive", and your home directory /home/myname will be placed on "data drive" because of the mount points.
As noted in the other answer, backup all your important data (documents, photos etc.) from your current Windows installation as they will be overwritten during Ubuntu installation.
