The minimum gain of the TIP42C is 15, so to get 6A, you will need 400ma of base current. That would require about a 27R base resistor. But that current is too much for a 2N3904.
A 2N2222 would work, and you would have to reduce it's base resistor to abo 680R. And both transistors are going to get hot. You will definitely need a heat sink on the TIP42C. It will be dropping about 1.5V at 6A thus dissapating 9 Watts.
A MOSFET would be a far better choice overall, and if it is just a bulb you are driving, use an N channel logic-level MOSFET and put it between the bulb and ground. That way it can be driven directly from the micro with no extra transistor.
Bob