The first is to search for the thread titled something like "How to get your TV card supported by MdiaPortal" and follow the instructions in that. Tis will get you going with the standard built in TV engine.
The second (which I would recommend) is to use the new TV Server system (look for download links and instructions on the MP Home Page). this is the new archittecture which has many advantages over the existing engine. It does take a bit more time to install, but will work out quicker than if you follow my suggestion #1.
The new TV engine should be able to work with any TV card that indows can see and access (which is nearly all of them) whereas support needs to be manually added for new cards with the existing engine.