Welcome aboard.
Take a look at the Dick O'Kane method and the John Cromwell method. Links are in the helpful hints/bag of tricks sticky. They are pretty easy to use.
Like Dogfish said, you need some vital pieces of information input into the TDC-range, speed, AOB, and bearing. There are numerous methods for gathering that information. Search around the forums for some of the more common.
The TDC is like any computer-garbage in, garbage out. This game is not like a point and shoot game. You need patience and time to develop your solutions. The more observations and the more accurate your observations and information, you'll hit more often.
Don't be afraid to ask more specific questions.