The Dreamcast and Sega Saturn Emulators require you to have a virtual drive in your computer that simulates you adding the games as if you were loading a disk. You need to download one like DAEMON Tools Pro, which is the best one I have found. It will create multiple drives in your computer, but be careful not to delete your own real drive if you ever want to delete the virtual ones, which look very similar. Once you do, you load the Dreamcast game into the drive through DAEMON's interface and then you start up NullDC, which will detect the game if you have configured the virtual drive in it's settings.
Although it sounds complicated, it is actually quite simple. You can find multiple easy to follow NullDC tutorials in YouTube that can help you install it, configure it and find a good virtual drive to work with it. Just type "NullDC tutorial" on the search box.
Also, keep in mind that not all games will work with NullDC or other Emulators, so you will need multiple ones and sometimes even multiple versions of the same Emulator depending on the game, since some of the updates make the new ones lose compatibility with some games. You will also find that some Emulators play some games slower than others. I suggest you try different Emulators if one specific game doesn't work.
I hope this helps...
