It depends. I think Zero999 has misunderstood you as you want 24V AC out.
If you use a bridge rectifier on one half of the total secondary, the resulting DC circuit *MUST* have no other common connection to the 24V AC circuit as its negative side swaps between the end of the winding feeding the bridge and the center tap with each half cycle.
Its probably better to make a full wave rectifier with two diodes, one from each end of the 24V winding, which gives you DC with respect to the center tap.
See Hammond Manufacturing Ltd's
Design Guide For Rectifier Use, the "FULL WAVE Capacitor Input Load" diagram.
N.B. for a light load, the DC voltage will be closer to 18V
* than 12V, so you will probably need to either regulate it down to 12V or put a dropper resistor in series with the relay coil.
Regarding Jonpaul's concerns: As you are connecting the windings in series getting the relative phasing wrong is non-hazardous. If you ge tit wrong, you will simply get a very low AC voltage where you expect 24V RMS, but each end to the center tap will still measure near 12V RMS. To fix this, swap the connections to ONE and only one of the secondaries.
If you were connecting them in parallel mis-phasing would cause a very large current to flow which could damage the transformer and would trip any reasonably rated circuit protection in the mains supply circuit.
*DONT* *DO* *THAT* !!!
* Q: Why 18V instead of the 17V Zero999 calculated?
A: Transformer nominal secondary voltage is specified at the secondary's full rated load current, and rises when less heavily loaded. The percentage the no load voltage is greater than the full load voltage by is known as the regulation factor, which is commonly in the 5% to 10% range for medium sized transformers.