Many of the new style prepared dinners are in paper or plastic trays whch are not oven-safe. If that is the case, transfer the food to an oven-safe dish before cooking. From there, it's not rocket science. Just use moderate heat until it's heated through. That would probably be 350-375 for 20-30-40 minutes from frozen (depending on the density of the food). If it's not ready, just put it back for 10 minutes at a time until it's heated.
Remember that a solid block of frozen food (like lasagne) takes longer to reheat than something loose (like veggie stir fry). Most (not all) tv dinners are already cooked, so you are just thawing and reheating them. You really can't screw it up - just heat and remove when hot.