Are you on a laptop, or desktop RedOktober99? Do you know what video card is in the computer? What kind of display do you have? Do you have another hard drive that you could move your Steam Library to? Where it is now will not affect it running correctly in Stock trim. You want to use that Uboat Missions expansion version, btw. The v1.3 from Steam is an odd beast of a different sort, and can be frustrating.
As cdrsubron7 said, you need the DirectX v9.0c that came with your Steam download. You also need to install the Windows Media 9 Codecs that are in with that, if they're listed separately in the Steam install. They might not be, but on a disk install, they are at the bottom of the list, after the DirectX, Adobe & Uplay choices. Uplay should not be on the Steam menu.
Are you logged into the Steam app on your computer when you attempt to start the game? You have to use the Steam Library menu at least the first time you start the game so that the license catches up with your computer.
As cdrsubron 7 says: "Welcome to SubSim!"
PS: cdrsubron7 is ~fast~ man - and good too!
