You didn't say how much current the relay coil draws, so I'm going to assume it's 20ma or less, although this will probably work for coil current up to 50ma or more.
Connect the emitter to ground, and the relay coil from +12v to the collector. Connect a 1N4001 across the coil, cathode to +12v.
Connect a 1k resistor between the base and the Q output of your 74121. Connect another 1k resistor from the Q output of your 74121 to the +5v supply that powers the 74121.
The reason for the last resistor (the pullup) is that TTL has very poor current sourcing capability, but very good sinking capability. This resistor supplies most of the base drive when the Q output is high, and the output can sink the 5ma through it when it is low.
I don't know of a tutorial page. Have you done a Google search for "transistor tutorial"?